Semiconductor device and display device

ABSTRACT

A semiconductor device with stable electrical characteristics is provided. The semiconductor device includes an oxide semiconductor film, a first gate electrode, a second gate electrode, a first conductive film, and a second conductive film. The first gate electrode is electrically connected to the second gate electrode. The first conductive film and the second conductive film function as a source electrode and a drain electrode. The oxide semiconductor film includes a first region that overlaps with the first conductive film, a second region that overlaps with the second conductive film, and a third region that overlaps with a gate electrode and the third conductive film. The first region includes a first edge that is opposed to the second region. The second region includes a second edge that is opposed to the first region. The length of the first edge is shorter than the length of the second edge.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a semiconductor device and a display device including the semiconductor device.

Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, and a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, and a composition of matter. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a storage device, a driving method thereof, and a manufacturing method thereof.

In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A semiconductor element such as a transistor, a semiconductor circuit, an arithmetic device, and a storage device are each an embodiment of a semiconductor device. An imaging device, a display device, a liquid crystal display device, a light-emitting device, an electro-optical device, a power generation device (including a thin film solar cell, an organic thin film solar cell, and the like), and an electronic appliance may each include a semiconductor device.

2. Description of the Related Art

Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a field-effect transistor (FET) or a thin film transistor (TFT)). Such transistors are applied to a wide range of electronic appliances such as an integrated circuit (IC) and an image display device (display device). A semiconductor material typified by silicon is widely known as a material for a semiconductor thin film that can be used in a transistor. As another material, an oxide semiconductor has been attracting attention (Patent Document 1).

It has been reported that a Corbino TFT in which a drain electrode concentrically surrounds a source electrode is fabricated with an oxide semiconductor, and the TFT is used as a driving TFT in an organic electroluminescent (EL) display, whereby a display device that is less likely to cause variations in luminance can be fabricated (Non-Patent Document 1).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.     2007-123861

Non-Patent Document

-   [Non-Patent Document 1] M. Mativenga et al., “Corbino TFTs for     Large-Area AMOLED Displays”, SID International Symposium Digest of     Technical Papers, 49.2 (2014), pp. 705-708

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide a transistor in which drain current is constant in a saturation region. Another object of one embodiment of the present invention is to provide a transistor having high on-state current. Another object of one embodiment of the present invention is to provide a display device that is less likely to cause variations in luminance. Another object of one embodiment of the present invention is to provide a novel semiconductor device. Another object of one embodiment of the present invention is to provide a novel display device.

Note that the description of a plurality of objects does not mutually preclude the existence. One embodiment of the present invention does not necessarily achieve all the objects listed above. Objects other than those listed above are apparent from the description of the specification, drawings, and claims, and such objects could be an object of one embodiment of the present invention.

One embodiment of the present invention is a semiconductor device including an oxide semiconductor film, a gate electrode, a first insulating film, a second insulating film, a first conductive film, a second conductive film, and a third conductive film. The oxide semiconductor film is provided over the gate electrode with the first insulating film positioned therebetween. The third conductive film is provided over the oxide semiconductor film with the second insulating film positioned therebetween. The first conductive film and the second conductive film are in contact with an upper surface of the oxide semiconductor film. The third conductive film is electrically connected to the gate electrode. The oxide semiconductor film includes a first region overlapping with the first conductive film, a second region overlapping with the second conductive film, and a third region overlapping with the gate electrode and the third conductive film. The first region includes a first edge that is opposed to the second region. The second region includes a second edge that is opposed to the first region. The length of the first edge is preferably shorter than the length of the second edge when seen from the above.

In the above structure, it is preferable that the second conductive film surround the first conductive film and that the conductive films be arranged in a circular pattern when seen from the above.

In the above structure, the oxide semiconductor film preferably includes a region with a thickness of greater than 0 nm and less than or equal to 20 nm.

In the above structure, the oxide semiconductor film preferably includes a plurality of crystal parts with c-axis alignment.

In the above structure, it is preferable that the oxide semiconductor film include In, M, and Zn (M is Ti, Ga, Y, Zr, Sn, La, Ce, Nd, or Hf) and include a region in which the content of In is larger than the content of M.

One embodiment of the present invention is a display device including a transistor and a pixel electrode electrically connected to the transistor. The transistor preferably includes the above-described semiconductor device.

One embodiment of the present invention is an electronic device including the above-described display device.

One embodiment of the present invention is an electronic device that includes the above-described display device and at least one of a microphone, a speaker, and an operation key.

In one embodiment of the present invention, a transistor in which drain current is constant in a saturation region can be provided. In one embodiment of the present invention, a transistor having high on-state current can be provided. In one embodiment of the present invention, a display device that is less likely to cause variations in luminance can be provided. In one embodiment of the present invention, a novel semiconductor device can be provided. In one embodiment of the present invention, a novel display device can be provided.

Note that the description of these effects does not disturb the existence of other effects. One embodiment of the present invention does not necessarily achieve all the effects listed above. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 2A to 2C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 3A to 3C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 4A to 4C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 5A and 5D are top views and FIGS. 5B and 5C are cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 6A to 6C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 7A and 7D are top views and FIGS. 7B and 7C are cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 8A to 8D are cross-sectional views illustrating embodiments of a semiconductor device.

FIGS. 9A and 9B are band diagrams.

FIGS. 10A to 10H are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 11A to 11F are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 12A to 12H are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 13A to 13D are Cs-corrected high-resolution TEM images of a cross section of a CAAC-OS and a cross-sectional schematic view of a CAAC-OS.

FIGS. 14A to 14D are Cs-corrected high-resolution TEM images of a plane of a CAAC-OS.

FIGS. 15A to 15C show structural analysis of a CAAC-OS and a single crystal oxide semiconductor by XRD.

FIGS. 16A and 16B show electron diffraction patterns of a CAAC-OS.

FIG. 17 shows a change of crystal parts of an In—Ga—Zn oxide owing to electron irradiation.

FIGS. 18A and 18B are schematic views illustrating deposition models of a CAAC-OS and an nc-OS.

FIGS. 19A to 19C illustrate an InGaZnO₄ crystal and a pellet.

FIGS. 20A to 20D are schematic views illustrating a deposition model of a CAAC-OS.

FIG. 21 is a top view illustrating one embodiment of a display device.

FIG. 22 is a cross-sectional view illustrating one embodiment of a display device.

FIG. 23 is a cross-sectional view illustrating one embodiment of a display device.

FIGS. 24A and 24B illustrate examples of a pixel configuration of one embodiment of a display device.

FIGS. 25A and 25B illustrate examples of a pixel configuration of one embodiment of a display device.

FIGS. 26A to 26C are a block diagram and circuit diagrams illustrating a display device.

FIGS. 27A and 27B are circuit diagrams illustrating a pixel of a display device.

FIG. 28 illustrates a display module.

FIGS. 29A to 29G illustrate electronic devices.

FIG. 30 shows Vd-Id characteristics and Vg-Id characteristics of fabricated transistors.

FIG. 31 shows Vd-Id characteristics and Vg-Id characteristics of fabricated transistors.

FIG. 32 shows Vd-Id characteristics and Vg-Id characteristics of fabricated transistors.

FIG. 33 shows Vd-Id characteristics and Vg-Id characteristics of fabricated transistors.

FIG. 34 shows Vd-Id characteristics and Vg-Id characteristics of fabricated transistors.

FIG. 35 shows Vd-Id characteristics of fabricated transistors.

FIG. 36 shows Vd-Id characteristics of fabricated transistors.

FIGS. 37A and 37B are top views showing regions in a semiconductor.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described below with reference to drawings. Note that the embodiments can be implemented with various modes. It will be readily appreciated by those skilled in the art that modes and details can be changed in various ways without departing from the spirit and scope of the present invention. Therefore, the present invention should not be interpreted as being limited to the following description of the embodiments.

In the drawings, the size, the layer thickness, and the region are exaggerated for clarity in some cases. Therefore, embodiments of the present invention are not limited to such a scale. Note that the drawings are schematic views showing ideal examples, and embodiments of the present invention are not limited to shapes or values shown in the drawings.

Note that in this specification, ordinal numbers such as “first”, “second”, and “third” are used in order to avoid confusion among components, and the terms do not limit the components numerically.

In this specification, terms for describing arrangement, such as “over”, “above”, “under”, and “below”, are used for convenience in describing a positional relation between components with reference to drawings. Furthermore, the positional relation between components is changed as appropriate in accordance with a direction in which each component is described. Thus, there is no limitation on terms used in this specification, and description can be made appropriately depending on the situation.

In this specification and the like, a transistor is an element having at least three terminals of a gate, a drain, and a source. The transistor has a channel region between a drain (a drain terminal, a drain region, or a drain electrode) and a source (a source terminal, a source region, or a source electrode), and current can flow through the drain, the channel region, and the source. Note that in this specification and the like, a channel region refers to a region through which current mainly flows.

Furthermore, functions of a source and a drain might be switched when transistors having different polarities are employed or a direction of current flow is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be switched in this specification and the like.

In this specification and the like, the expression “electrically connected” includes the case where components are connected through an “object having any electric function”. There is no particular limitation on an “object having any electric function” as long as electric signals can be transmitted and received between components that are connected through the object. Examples of an “object having any electric function” are a switching element such as a transistor, a resistor, an inductor, a capacitor, and elements with a variety of functions as well as an electrode and a wiring.

In this specification and the like, a “silicon oxynitride film” refers to a film that includes oxygen at a higher proportion than nitrogen, and a “silicon nitride oxide film” refers to a film that includes nitrogen at a higher proportion than oxygen.

In describing structures of the present invention with reference to the drawings in this specification and the like, common reference numerals are used for the same portions in different drawings.

In this specification and the like, a term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly includes the case where the angle is greater than or equal to −5° and less than or equal to 5°. In addition, a term “perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly includes the case where the angle is greater than or equal to 85° and less than or equal to 95°.

In this specification and the like, the terms “film” and “layer” can be switched depending on the case or circumstances. For example, the term “conductive layer” can be used instead of the term “conductive film” in some cases. Similarly, the term “insulating film” can be used instead of the term “insulating layer” in some cases.

Embodiment 1

In this embodiment, a semiconductor device of one embodiment of the present invention and a manufacturing method thereof are described with reference to FIGS. 1A to 1C, FIGS. 2A to 2C, FIGS. 3A to 3C, FIGS. 4A to 4C, FIGS. 5A to 5D, FIGS. 6A to 6C, FIGS. 7A to 7D, FIGS. 8A to 8D, FIGS. 9A and 9B, FIGS. 10A to 10H, FIGS. 11A to 11F, and FIGS. 12A to 12H.

Structure Example 1 of Semiconductor Device

FIG. 1A is a top view of a transistor 100 that is a semiconductor device of one embodiment of the present invention. FIG. 1B is a cross-sectional view taken along a dashed-dotted line X1-X2 in FIG. 1A, and FIG. 1C is a cross-sectional view taken along a dashed-dotted line Y1-Y2 in FIG. 1A. Note that in FIG. 1A, some components of the transistor 100 (e.g., an insulating film serving as a gate insulating film) are not illustrated to avoid complexity. The direction of the dashed-dotted line X1-X2 may be called a channel length direction, and the direction of the dashed-dotted line Y1-Y2 may be called a channel width direction. As in FIG. 1A, some components are not illustrated in some cases in top views of transistors described below.

The transistor 100 includes a conductive film 104 that functions as a gate electrode over a substrate 102; an insulating film 106 over the substrate 102 and the conductive film 104; an insulating film 107 over the insulating film 106; an oxide semiconductor film 108 over the insulating film 107; a conductive film 112 a that is electrically connected to the oxide semiconductor film 108 and that functions as one of a source electrode and a drain electrode; a conductive film 112 b that is electrically connected to the oxide semiconductor film 108 and that functions as the other of the source electrode and the drain electrode; an insulating film 114 over the oxide semiconductor film 108 and the conductive films 112 a and 112 b; an insulating film 116 over the insulating film 114; an insulating film 118 over the insulating film 116; and conductive films 120 a and 120 b over the insulating film 118. The conductive film 120 a is electrically connected to the conductive film 112 b through an opening 142 c provided in the insulating films 114, 116, and 118.

In FIGS. 1A to 1C, L1 indicates the channel length of the transistor 100. Here, the channel length L1 refers to a distance between an edge of the conductive film 112 a and an edge of the conductive film 112 b when seen from above the oxide semiconductor film 108. The channel length L1 is greater than or equal to 1 μm and less than or equal to 100 μm, or greater than or equal to 1 μm and less than or equal to 30 μm. In one transistor, channel lengths in all regions are not necessarily the same. In other words, the channel length of one transistor is not limited to one value in some cases. Therefore, in this specification, the channel length is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed.

In FIG. 1A, W1 indicates the channel width of the transistor 100. Here, the channel width W1 refers to a length of a portion where the conductive film 112 a and the conductive film 112 b are opposed to each other when seen from above the oxide semiconductor film 108. In one transistor, channel widths in all regions do not necessarily have the same value. In other words, the channel width of one transistor is not limited to one value in some cases. Therefore, in this specification, the channel width is any one of values, the maximum value, the minimum value, or the average value in a region where a channel is formed.

The insulating films 106 and 107 function as a first gate insulating film of the transistor 100. The insulating films 114 and 116 include oxygen and have a function of supplying oxygen to the oxide semiconductor film 108. The insulating films 114, 116, and 118 function as a second gate insulating film of the transistor 100. The insulating film 118 functions as a protective insulating film that inhibits entry of impurities into the transistor 100. The conductive film 120 a functions as, for example, a pixel electrode used in a display device. The conductive film 120 b functions as a second gate electrode (also referred to as a back gate electrode).

As illustrated in FIG. 1C, the conductive film 120 b is electrically connected to the conductive film 104 that functions as a first gate electrode, through openings 142 a and 142 b provided in the insulating films 106, 107, 114, 116, and 118. Accordingly, the conductive film 120 b and the conductive film 104 are supplied with the same potential.

When oxygen vacancy is formed in the oxide semiconductor film 108 included in the transistor 100, electrons serving as carriers are generated; as a result, the transistor 100 tends to be normally-on. Therefore, for stable transistor characteristics, it is preferred to reduce oxygen vacancy in the oxide semiconductor film 108. In the structure of the transistor of one embodiment of the present invention, excess oxygen is introduced into the insulating film over the oxide semiconductor film 108, here, the insulating film 114 over the oxide semiconductor film 108, whereby oxygen is moved from the insulating film 114 to the oxide semiconductor film 108 to fill oxygen vacancy in the oxide semiconductor film 108. Alternatively, excess oxygen is introduced into the insulating film 116 over the oxide semiconductor film 108, whereby oxygen is moved from the insulating film 116 to the oxide semiconductor film 108 through the insulating film 114 to fill oxygen vacancy in the oxide semiconductor film 108. Alternatively, excess oxygen is introduced into the insulating films 114 and 116 over the oxide semiconductor film 108, whereby oxygen is moved from both the insulating films 114 and 116 to the oxide semiconductor film 108 to fill oxygen vacancy in the oxide semiconductor film 108.

It is preferable that the insulating films 114 and 116 each include a region (oxygen excess region) including oxygen in excess of that in the stoichiometric composition. In other words, the insulating films 114 and 116 are preferably insulating films capable of releasing oxygen. Note that the oxygen excess region is formed in each of the insulating films 114 and 116 in such a manner that oxygen is introduced into the formed insulating films 114 and 116, for example. As a method for introducing oxygen, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like may be employed.

The oxide semiconductor film 108 includes oxygen, In, and Zn. A metal M (M is Ti, Ga, Y, Zr, Sn, La, Ce, Nd, or Hf) may be further included. Typically, an In—Ga oxide, an In—Zn oxide, or an In-M-Zn oxide can be used for the oxide semiconductor film 108. It is particularly preferable to use an In-M-Zn oxide for the oxide semiconductor film 108.

The oxide semiconductor film 108 preferably includes oxygen, In, and Ga. In that case, it is preferable that the oxide semiconductor film 108 have a homologous structure and the content of In be larger than that of Ga. The oxide semiconductor film 108 in which the content of In is larger than that of Ga can increase the field-effect mobility (also simply referred to as mobility or μFE) of the transistor 100. Specifically, the field-effect mobility of the transistor 100 can exceed 10 cm²/Vs.

For example, the use of the transistor with high field-effect mobility for a gate driver that generates a gate signal (specifically, a demultiplexer connected to an output terminal of a shift register included in a gate driver) allows a semiconductor device or a display device to have a narrow frame.

However, the electrical characteristics of the transistor 100 including the oxide semiconductor film 108 in which the content of In is larger than that of Ga are likely to be changed. Specifically, when the above-described oxide semiconductor film 108 is used for a channel region of the transistor 100, the transistor 100 is likely to be affected by the channel length modulation effect, and in some cases, the drain current does not become constant (the transistor does not operate in the saturation region or the saturation characteristics of the transistor cannot be obtained) in the Vd (drain voltage) vs. Id (drain current) characteristics even when the drain voltage reaches a pinch-off voltage.

However, in the semiconductor device of one embodiment of the present invention, the thickness of the oxide semiconductor film 108 can be less than or equal to 35 nm, less than or equal to 20 nm, or less than or equal to 10 nm. The thickness of the oxide semiconductor film 108 is greater than or equal to 3 nm and less than or equal to 35 nm, preferably greater than or equal to 3 nm and less than or equal to 20 nm, further preferably greater than or equal to 3 nm and less than or equal to 10 nm. In other words, the oxide semiconductor film 108 includes a region with a thickness of greater than 0 nm and less than or equal to 35 nm.

In other words, the channel formation region in the oxide semiconductor film 108 includes a region with a thickness of greater than 0 nm and less than or equal to 35 nm or a region with a thickness of greater than 0 nm and less than 20 nm. The channel formation region preferably includes a region with a thickness of greater than or equal to 3 nm and less than or equal to 35 nm, a region with a thickness of greater than or equal to 3 nm and less than or equal to 20 nm, or a region greater than or equal to 3 nm and less than or equal to 10 nm.

In other words, the oxide semiconductor film 108 in a portion which overlaps with neither the conductive film 112 a nor the conductive film 112 b and overlaps with the conductive film 104 includes a region with a thickness of greater than 0 nm and less than or equal to 35 nm or a region with a thickness of greater than 0 nm and less than 20 nm. The portion preferably includes a region with a thickness of greater than or equal to 3 nm and less than or equal to 35 nm, a region with a thickness of greater than or equal to 3 nm and less than or equal to 20 nm, or a region greater than or equal to 3 nm and less than or equal to 10 nm.

Owing to a region with a thickness in the above range, even when the oxide semiconductor film 108 has a content of In larger than a content of Ga, the transistor 100 can operate in the saturation region.

As illustrated in FIG. 1B, the oxide semiconductor film 108 is positioned to face the conductive film 104 that functions as a first gate electrode and the conductive film 120 b that functions as the second gate electrode, and is sandwiched between the two conductive films which function as gate electrodes. The length in the channel length direction and the length in the channel width direction of the conductive film 120 b that functions as the second gate electrode are longer than the length in the channel length direction and the length in the channel width direction of the oxide semiconductor film 108. The whole oxide semiconductor film 108 is covered with the conductive film 120 b with the insulating films 114, 116, and 118 positioned therebetween. Since the conductive film 120 b that functions as the second gate electrode is connected to the conductive film 104 that functions as a first gate electrode through the openings 142 a and 142 b provided in the insulating films 106, 107, 114, 116, and 118, a side surface of the oxide semiconductor film 108 in the channel width direction faces the conductive film 120 b that functions as the second gate electrode with the insulating films 114, 116, and 118 positioned therebetween.

In other words, in the channel width direction of the transistor 100, the conductive film 104 that functions as the first gate electrode and the conductive film 120 b that functions as the second gate electrode are connected to each other through the openings provided in the insulating films 106 and 107 which function as the first gate insulating film and the insulating films 114, 116, and 118 which function as the second gate insulating film; and the conductive film 104 and the conductive film 120 b surround the oxide semiconductor film 108 with the insulating films 106 and 107 which function as the first gate insulating film and the insulating films 114, 116, and 118 which function as the second gate insulating film positioned therebetween.

With such a structure, the oxide semiconductor film 108 included in the transistor 100 can be electrically surrounded by electric fields of the conductive film 104 that functions as the first gate electrode and the conductive film 120 b that functions as the second gate electrode. A device structure of a transistor, like the structure of the transistor 100, in which electric fields of a first gate electrode and a second gate electrode electrically surround an oxide semiconductor film where a channel region is formed can be referred to as a surrounded channel (s-channel) structure.

Since the transistor 100 has the s-channel structure, an electric field for inducing a channel can be effectively applied to the oxide semiconductor film 108 by the conductive film 104 that functions as the first gate electrode; therefore, the current drive capability of the transistor 100 can be improved and high on-state current characteristics can be obtained. Since the on-state current can be increased, the size of the transistor 100 can be reduced. In addition, since the transistor 100 is surrounded by the conductive film 104 that functions as the first gate electrode and the conductive film 120 b that functions as the second gate electrode, the mechanical strength of the transistor 100 can be increased.

Other components of the semiconductor device of this embodiment are described in detail below.

Substrate

There is no particular limitation on the property of a material and the like of the substrate 102 as long as the material has heat resistance enough to withstand at least heat treatment to be performed later. For example, a glass substrate, a ceramic substrate, a quartz substrate, or a sapphire substrate may be used as the substrate 102. Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like, a compound semiconductor substrate made of silicon germanium or the like, an SOI substrate, or the like may be used as the substrate 102. Still alternatively, any of these substrates provided with a semiconductor element may be used as the substrate 102. In the case where a glass substrate is used as the substrate 102, a glass substrate having any of the following sizes can be used: the 6th generation (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm) Thus, a large-sized display device can be fabricated.

Alternatively, a flexible substrate may be used as the substrate 102, and the transistor 100 may be provided directly on the flexible substrate. Alternatively, a separation layer may be provided between the substrate 102 and the transistor 100. The separation layer can be used when part or the whole of a semiconductor device formed over the separation layer is separated from the substrate 102 and transferred onto another substrate. In such a case, the transistor 100 can be transferred to a substrate having low heat resistance or a flexible substrate as well.

First Gate Electrode, Source Electrode, and Drain Electrode

The conductive film 104 that functions as the first gate electrode and the conductive films 112 a and 112 b which function as source and drain electrodes can each be formed using a metal element selected from chromium (Cr), copper (Cu), aluminum (Al), gold (Au), silver (Ag), zinc (Zn), molybdenum (Mo), tantalum (Ta), titanium (Ti), tungsten (W), manganese (Mn), nickel (Ni), iron (Fe), and cobalt (Co); an alloy including any of these metal element as its component; an alloy including a combination of any of these elements; or the like.

Furthermore, the conductive films 104, 112 a, and 112 b may have a single-layer structure or a stacked-layer structure of two or more layers. For example, a single-layer structure of an aluminum film including silicon, a two-layer structure in which a titanium film is stacked over an aluminum film, a two-layer structure in which a titanium film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a tantalum nitride film or a tungsten nitride film, or a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order can be employed. Alternatively, an alloy film or a nitride film in which aluminum and one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium are combined may be used.

The conductive films 104, 112 a, and 112 b can be formed using a light-transmitting conductive material such as indium tin oxide, indium oxide including tungsten oxide, indium zinc oxide including tungsten oxide, indium oxide including titanium oxide, indium tin oxide including titanium oxide, indium zinc oxide, or indium tin oxide to which silicon oxide is added.

A Cu—X alloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti) may be used for the conductive films 104, 112 a, and 112 b. Use of a Cu—X alloy film enables the manufacturing cost to be reduced because wet etching process can be used in the processing.

First Gate Insulating Film

As each of the insulating films 106 and 107 which function as the first gate insulating film of the transistor 100, an insulating layer including at least one of the following films formed by a plasma enhanced chemical vapor deposition (PECVD) method, a sputtering method, or the like can be used: a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, a silicon nitride film, an aluminum oxide film, a hafnium oxide film, an yttrium oxide film, a zirconium oxide film, a gallium oxide film, a tantalum oxide film, a magnesium oxide film, a lanthanum oxide film, a cerium oxide film, and a neodymium oxide film. Note that instead of a stacked-layer structure of the insulating films 106 and 107, an insulating film of a single layer formed using a material selected from the above or an insulating film of three or more layers may be used.

The insulating film 106 functions as a blocking film which keeps out oxygen. For example, in the case where excess oxygen is supplied to the insulating film 107, the insulating film 114, the insulating film 116, and/or the oxide semiconductor film 108, the insulating film 106 can keep out oxygen.

Note that the insulating film 107 that is in contact with the oxide semiconductor film 108 that functions as a channel region of the transistor 100 is preferably an oxide insulating film and preferably includes a region including oxygen in excess of the stoichiometric composition (oxygen-excess region). In other words, the insulating film 107 is an insulating film which is capable of releasing oxygen. In order to provide the oxygen excess region in the insulating film 107, the insulating film 107 is formed in an oxygen atmosphere, for example. Alternatively, the oxygen excess region may be formed by introduction of oxygen into the insulating film 107 after the deposition. As a method for introducing oxygen, an ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like may be employed.

In the case where hafnium oxide is used for the insulating film 107, the following effect is attained. Hafnium oxide has a higher dielectric constant than silicon oxide and silicon oxynitride. Therefore, the insulating film 107 using hafnium oxide can have a larger thickness than the insulating film 107 using silicon oxide; thus, leakage current due to tunnel current can be low. That is, it is possible to provide a transistor with a low off-state current. Moreover, hafnium oxide with a crystalline structure has higher dielectric constant than hafnium oxide with an amorphous structure. Therefore, it is preferable to use hafnium oxide with a crystalline structure in order to provide a transistor with a low off-state current. Examples of the crystalline structure include a monoclinic crystal structure and a cubic crystal structure. Note that one embodiment of the present invention is not limited thereto.

In this embodiment, a silicon nitride film is formed as the insulating film 106, and a silicon oxide film is formed as the insulating film 107. The silicon nitride film has a higher dielectric constant than a silicon oxide film and needs a larger thickness for capacitance equivalent to that of the silicon oxide film. Thus, when the silicon nitride film is included in the gate insulating film of the transistor 100, the physical thickness of the insulating film can be increased. This makes it possible to reduce a decrease in withstand voltage of the transistor 100 and furthermore to increase the withstand voltage, thereby reducing electrostatic discharge damage to the transistor 100.

Oxide Semiconductor Film

An In—Ga oxide, an In—Zn oxide, or an In-M-Zn oxide can be used for the oxide semiconductor film 108. It is particularly preferable to use an In-M-Zn oxide for the oxide semiconductor film 108.

In the case where the oxide semiconductor film 108 is formed of In-M-Zn oxide, it is preferable that the atomic ratio of metal elements of a sputtering target used for forming the In-M-Zn oxide satisfy In≧M and Zn≧M. As the atomic ratio of metal elements of such a sputtering target, In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=2:1:3, In:M:Zn=3:1:2, and In:M:Zn=4:2:4.1 are preferable. In the case where the oxide semiconductor film 108 is formed of In-M-Zn oxide, it is preferable to use a target including polycrystalline In-M-Zn oxide as the sputtering target. The use of the target including polycrystalline In-M-Zn oxide facilitates formation of the oxide semiconductor film 108 having crystallinity. Note that the atomic ratios of metal elements in the formed oxide semiconductor film 108 vary from the above atomic ratio of metal elements of the sputtering target within a range of ±40% as an error. For example, when a sputtering target with an atomic ratio of In to Ga and Zn of 4:2:4.1 is used, the atomic ratio of In to Ga and Zn in the oxide semiconductor film 108 may be 4:2:3 or in the vicinity of 4:2:3.

Note that in the case where the oxide semiconductor film 108 is formed of In-M-Zn oxide, the proportion of In and the proportion of M, not taking Zn and O into consideration, are preferably greater than 25 atomic % and less than 75 atomic %, respectively, and more preferably greater than 34 atomic % and less than 66 atomic %, respectively.

The energy gap of the oxide semiconductor film 108 is 2 eV or more, preferably 2.5 eV or more, or further preferably 3 eV or more. With the use of an oxide semiconductor having such a wide energy gap, the off-state current of the transistor 100 can be reduced.

The thickness of the oxide semiconductor film 108 is greater than or equal to 3 nm and less than or equal to 35 nm, preferably greater than or equal to 3 nm and less than or equal to 20 nm, or further preferably greater than or equal to 3 nm and less than or equal to 10 nm.

An oxide semiconductor film with low carrier density is used as the oxide semiconductor film 108. For example, an oxide semiconductor film whose carrier density is lower than or equal to 1×10¹⁷/cm³, preferably lower than or equal to 1×10¹⁵/cm³, further preferably lower than or equal to 1×10¹³/cm³, still further preferably lower than or equal to 1×10¹¹/cm³ is used as the oxide semiconductor film 108.

Note that, without limitation to the compositions and materials described above, a material with an appropriate composition may be used depending on required semiconductor characteristics and electrical characteristics (e.g., field-effect mobility and threshold voltage) of a transistor. Furthermore, in order to obtain required semiconductor characteristics of a transistor, it is preferable that the carrier density, the impurity concentration, the defect density, the atomic ratio of a metal element to oxygen, the interatomic distance, the density, and the like of the oxide semiconductor film 108 be set to be appropriate.

Note that it is preferable to use, as the oxide semiconductor film 108, an oxide semiconductor film in which the impurity concentration is low and density of defect states is low, in which case the transistor can have more excellent electrical characteristics. Here, the state in which impurity concentration is low and density of defect states is low (the amount of oxygen vacancy is small) is referred to as “highly purified intrinsic” or “substantially highly purified intrinsic”. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources, and thus can have a low carrier density. Thus, a transistor in which a channel region is formed in the oxide semiconductor film rarely has a negative threshold voltage (is rarely normally on). A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and accordingly has few carrier traps in some cases. Furthermore, the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has an extremely low off-state current; even when an element has a channel width W of 1×10⁶ μm and a channel length L of 10 μm, the off-state current can be less than or equal to the measurement limit of a semiconductor parameter analyzer, i.e., less than or equal to 1×10⁻¹³ A, at a voltage (drain voltage) between a source electrode and a drain electrode of from 1 V to 10 V.

Accordingly, the transistor in which the channel region is formed in the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film can have a small variation in electrical characteristics and high reliability. Charges trapped by the trap states in the oxide semiconductor film take a long time to be released and may behave like fixed charges. Thus, the transistor whose channel region is formed in the oxide semiconductor film having a high density of trap states has unstable electrical characteristics in some cases. As examples of the impurities, hydrogen, nitrogen, alkali metal, alkaline earth metal, and the like are given.

Hydrogen included in the oxide semiconductor film reacts with oxygen bonded to a metal atom to be water, and also causes oxygen vacancy in a lattice from which oxygen is released (or a portion from which oxygen is released). Due to entry of hydrogen into the oxygen vacancy, an electron serving as a carrier is generated in some cases. Furthermore, in some cases, bonding of part of hydrogen to oxygen bonded to a metal element causes generation of an electron serving as a carrier. Thus, a transistor including an oxide semiconductor film which contains hydrogen is likely to be normally on. Accordingly, it is preferable that hydrogen be reduced as much as possible in the oxide semiconductor film 108. Specifically, in the oxide semiconductor film 108, the concentration of hydrogen which is measured by secondary ion mass spectrometry (SIMS) is lower than or equal to 2×10²⁰ atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, further preferably lower than or equal to 1×10¹⁹ atoms/cm³, further preferably lower than or equal to 5×10¹⁸ atoms/cm³, further preferably lower than or equal to 1×10¹⁸ atoms/cm³, further preferably lower than or equal to 5×10¹⁷ atoms/cm³, or further preferably lower than or equal to 1×10¹⁶ atoms/cm³.

When silicon or carbon that is one of elements belonging to Group 14 is included in the oxide semiconductor film 108, oxygen vacancy is increased in the oxide semiconductor film 108, and the oxide semiconductor film 108 becomes an n-type film. Thus, the concentration of silicon or carbon (the concentration measured by SIMS) in the oxide semiconductor film 108 or the concentration of silicon or carbon (the concentration measured by SIMS) at or near an interface with the oxide semiconductor film 108 is set to be lower than or equal to 2×10¹⁸ atoms/cm³, or preferably lower than or equal to 2×10¹⁷ atoms/cm³.

In addition, the concentration of alkali metal or alkaline earth metal of the oxide semiconductor film 108, which is measured by SIMS, is lower than or equal to 1×10¹⁸ atoms/cm³, or preferably lower than or equal to 2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metal might generate carriers when bonded to an oxide semiconductor, in which case the off-state current of the transistor might be increased. Therefore, it is preferable to reduce the concentration of alkali metal or alkaline earth metal of the oxide semiconductor film 108.

Furthermore, when including nitrogen, the oxide semiconductor film 108 easily becomes n-type by generation of electrons serving as carriers and an increase of carrier density. Thus, a transistor including an oxide semiconductor film which contains nitrogen is likely to have normally-on characteristics. For this reason, nitrogen in the oxide semiconductor film is preferably reduced as much as possible; the concentration of nitrogen which is measured by SIMS is preferably set to be, for example, lower than or equal to 5×10¹⁸ atoms/cm³.

The oxide semiconductor film 108 may have a non-single-crystal structure, for example. The non-single crystal structure includes a c-axis aligned crystalline oxide semiconductor (CAAC-OS) which is described later, a polycrystalline structure, a microcrystalline structure, or an amorphous structure, for example. Among the non-single crystal structure, the amorphous structure has the highest density of defect states, whereas CAAC-OS has the lowest density of defect states.

The oxide semiconductor film 108 may have an amorphous structure, for example. The oxide semiconductor films having the amorphous structure each have disordered atomic arrangement and no crystalline component, for example. Alternatively, the oxide films having an amorphous structure have, for example, an absolutely amorphous structure and no crystal part.

Note that the oxide semiconductor film 108 may be a mixed film including two or more of the following: a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a region of CAAC-OS, and a region having a single-crystal structure. The mixed film has a single-layer structure including, for example, two or more of a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single-crystal structure in some cases. Furthermore, in some cases, the mixed film has a stacked-layer structure of two or more of a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single-crystal structure.

Insulating Film Functioning as Second Gate Insulating Film

The insulating films 114 and 116 each have a function of supplying oxygen to the oxide semiconductor film 108. The insulating film 118 functions as a protective insulating film for the transistor 100. The insulating films 114 and 116 include oxygen. The insulating film 114 is an insulating film that is permeable to oxygen. Note that the insulating film 114 also functions as a film that relieves damage to the oxide semiconductor film 108 at the time of forming the insulating film 116 in a later step.

A silicon oxide film, a silicon oxynitride film, or the like with a thickness greater than or equal to 5 nm and less than or equal to 150 nm, or preferably greater than or equal to 5 nm and less than or equal to 50 nm, can be used as the insulating film 114.

In addition, it is preferable that the number of defects in the insulating film 114 be small and typically the spin density corresponding to a signal that appears at g=2.001 due to a dangling bond of silicon be lower than or equal to 3×10¹⁷ spins/cm³ by electron spin resonance (ESR) measurement. This is because if the density of defects in the insulating film 114 is high, oxygen is bonded to the defects and the amount of oxygen that permeates through the insulating film 114 is decreased.

Note that all oxygen entering the insulating film 114 from the outside does not move to the outside of the insulating film 114 and some oxygen remains in the insulating film 114. Furthermore, movement of oxygen occurs in the insulating film 114 in some cases in such a manner that oxygen enters the insulating film 114 and oxygen included in the insulating film 114 moves to the outside of the insulating film 114. When an oxide insulating film which is permeable to oxygen is formed as the insulating film 114, oxygen released from the insulating film 116 provided over the insulating film 114 can be moved to the oxide semiconductor film 108 through the insulating film 114.

Note that the insulating film 114 can be formed using an oxide insulating film having a low density of states due to nitrogen oxide. Note that the density of states due to nitrogen oxide can be formed between the energy of the valence band maximum (E_(v) _(—) _(os)) and the energy of the conduction band minimum (E_(c) _(—) _(os)) of the oxide semiconductor film. A silicon oxynitride film that releases less nitrogen oxide, an aluminum oxynitride film that releases less nitrogen oxide, and the like can be used as the above oxide insulating film.

Note that a silicon oxynitride film that releases less nitrogen oxide is a film of which the amount of released ammonia is larger than the amount of released nitrogen oxide in thermal desorption spectroscopy analysis; the amount of released ammonia is typically greater than or equal to 1×10¹⁸/cm³ and less than or equal to 5×10¹⁹/cm³. Note that the amount of released ammonia is the amount of ammonia released by heat treatment with which the surface temperature of a film becomes higher than or equal to 50° C. and lower than or equal to 650° C., or preferably higher than or equal to 50° C. and lower than or equal to 550° C.

Nitrogen oxide (NO_(x); x is greater than or equal to 0 and less than or equal to 2, or preferably greater than or equal to 1 and less than or equal to 2), typically NO₂ or NO, forms levels in the insulating film 114, for example. The level is positioned in the energy gap of the oxide semiconductor film 108. Therefore, when nitrogen oxide is diffused to the interface between the insulating film 114 and the oxide semiconductor film 108, an electron is in some cases trapped by the level on the insulating film 114 side. As a result, the trapped electron remains at or near the interface between the insulating film 114 and the oxide semiconductor film 108; thus, the threshold voltage of the transistor is shifted in the positive direction.

Nitrogen oxide reacts with ammonia and oxygen in heat treatment. Since nitrogen oxide included in the insulating film 114 reacts with ammonia included in the insulating film 116 in heat treatment, nitrogen oxide included in the insulating film 114 is reduced. Therefore, an electron is hardly trapped at the interface between the insulating film 114 and the oxide semiconductor film 108.

By using, for the insulating film 114, the above-described oxide insulating film, the shift in the threshold voltage of the transistor can be reduced.

Note that in an ESR spectrum at 100 K or lower of the insulating film 114, by heat treatment of a manufacturing process of the transistor, typically heat treatment at a temperature higher than or equal to 300° C. and lower than the strain point of the substrate, a first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, a second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and a third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 are observed. The split width of the first and second signals and the split width of the second and third signals that are obtained by ESR measurement using an X-band are each approximately 5 mT. The sum of the spin densities of the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 is lower than 1×10¹⁸ spins/cm³, typically higher than or equal to 1×10¹⁷ spins/cm³ and lower than 1×10¹⁸ spins/cm³.

In the ESR spectrum at 100 K or lower, the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 correspond to signals attributed to nitrogen oxide (NO_(x); x is greater than or equal to 0 and less than or equal to 2, preferably greater than or equal to 1 and less than or equal to 2). Typical examples of nitrogen oxide include nitrogen monoxide and nitrogen dioxide. In other words, the lower the total spin density of the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 is, the lower the content of nitrogen oxide in the oxide insulating film is.

The nitrogen concentration of the above-described oxide insulating film measured by SIMS is lower than or equal to 6×10²⁰ atoms/cm³.

The above-described oxide insulating film is formed by a PECVD method at a substrate temperature higher than or equal to 220° C., higher than or equal to 280° C., or higher than or equal to 350° C. using silane and dinitrogen monoxide, whereby a dense and hard film can be formed.

The insulating film 116 is formed using an oxide insulating film that includes oxygen in excess of that in the stoichiometric composition. Part of oxygen is released by heating from the oxide insulating film including oxygen in excess of that in the stoichiometric composition. The oxide insulating film including oxygen in excess of that in the stoichiometric composition is an oxide insulating film of which the amount of released oxygen converted into oxygen atoms is greater than or equal to 1.0×10¹⁹ atoms/cm³, or preferably greater than or equal to 3.0×10²⁰ atoms/cm³, in TDS analysis. Note that the temperature of the film surface in the TDS analysis is preferably higher than or equal to 100° C. and lower than or equal to 700° C., or higher than or equal to 100° C. and lower than or equal to 500° C.

A silicon oxide film, a silicon oxynitride film, or the like with a thickness greater than or equal to 30 nm and less than or equal to 500 nm, or preferably greater than or equal to 50 nm and less than or equal to 400 nm, can be used as the insulating film 116.

It is preferable that the number of defects in the insulating film 116 be small, and typically the spin density corresponding to a signal which appears at g=2.001 due to a dangling bond of silicon be lower than 1.5×10¹⁸ spins/cm³, or further preferably lower than or equal to 1×10¹⁸ spins/cm³ by ESR measurement. Note that the insulating film 116 is provided more apart from the oxide semiconductor film 108 than the insulating film 114 is; thus, the insulating film 116 may have higher density of defects than the insulating film 114.

Furthermore, the insulating films 114 and 116 can be formed using insulating films formed of the same kinds of materials; thus, a boundary between the insulating films 114 and 116 cannot be clearly observed in some cases. Thus, in this embodiment, the boundary between the insulating films 114 and 116 is shown by a dashed line. Although a two-layer structure of the insulating films 114 and 116 is described in this embodiment, the present invention is not limited to this. For example, a single-layer structure of the insulating film 114 may be used.

The insulating film 118 includes nitrogen. Alternatively, the insulating film 118 includes nitrogen and silicon. The insulating film 118 has a function of blocking oxygen, hydrogen, water, alkali metal, alkaline earth metal, or the like. It is possible to prevent outward diffusion of oxygen from the oxide semiconductor film 108, outward diffusion of oxygen included in the insulating films 114 and 116, and entry of hydrogen, water, or the like into the oxide semiconductor film 108 from the outside by providing the insulating film 118. A nitride insulating film can be used as the insulating film 118, for example. As the nitride insulating film, a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, an aluminum nitride oxide film, and the like can be given. Note that instead of the nitride insulating film having a blocking effect against oxygen, hydrogen, water, alkali metal, alkaline earth metal, and the like, an oxide insulating film having a blocking effect against oxygen, hydrogen, water, and the like, may be provided. As the oxide insulating film having a blocking effect against oxygen, hydrogen, water, and the like, an aluminum oxide film, an aluminum oxynitride film, a gallium oxide film, a gallium oxynitride film, an yttrium oxide film, an yttrium oxynitride film, a hafnium oxide film, a hafnium oxynitride film, and the like can be given.

Although the variety of films such as the conductive films, the insulating films, and the oxide semiconductor film which are described above can be formed by a sputtering method or a PECVD method, such films may be formed by another method, e.g., a thermal CVD method. As an example of a thermal CVD method, a metal organic chemical vapor deposition (MOCVD) method and an atomic layer deposition (ALD) method can be given.

A thermal CVD method has an advantage that no defect due to plasma damage is generated because it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a manner that a source gas and an oxidizer are supplied to a chamber at a time so that the pressure in the chamber is set to an atmospheric pressure or a reduced pressure, and react with each other in the vicinity of the substrate or over the substrate.

Deposition by an ALD method may be performed in such a manner that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, source gases for reaction are sequentially introduced into the chamber, and then the sequence of the gas introduction is repeated. For example, two or more kinds of source gases are sequentially supplied to the chamber by switching respective switching valves (also referred to as high-speed valves). For example, a first source gas is introduced, an inert gas (e.g., argon or nitrogen) or the like is introduced at the same time as or after the introduction of the first gas so that the source gases are not mixed, and then a second source gas is introduced. Note that in the case where the first source gas and the inert gas are introduced at a time, the inert gas serves as a carrier gas, and the inert gas may also be introduced at the same time as the introduction of the second source gas. Alternatively, the first source gas may be exhausted by vacuum evacuation instead of the introduction of the inert gas, and then the second source gas may be introduced. The first source gas is adsorbed on the surface of the substrate to form a first layer; then the second source gas is introduced to react with the first layer; as a result, a second layer is stacked over the first layer, so that a thin film is formed. The sequence of the gas introduction is repeated plural times until a desired thickness is obtained, whereby a thin film with excellent step coverage can be formed. The thickness of the thin film can be adjusted by the number of repetition times of the sequence of the gas introduction; therefore, an ALD method makes it possible to accurately adjust a thickness and thus is suitable for manufacturing a minute FET.

The variety of films such as the conductive films, the insulating films, the oxide semiconductor films, and the metal oxide films which are described above can be formed by a thermal CVD method such as an MOCVD method or an ALD method. For example, in the case where an In—Ga—Zn—O film is formed, trimethylindium, trimethylgallium, and dimethylzinc are used. Note that the chemical formula of trimethylindium is In(CH₃)₃. The chemical formula of trimethylgallium is Ga(CH₃)₃. The chemical formula of dimethylzinc is Zn(CH₃)₂. Without limitation to the above combination, triethylgallium (chemical formula: Ga(C₂H₅)₃) can be used instead of trimethylgallium and diethylzinc (chemical formula: Zn(C₂H₅)₂) can be used instead of dimethylzinc.

For example, in the case where a hafnium oxide film is formed by a deposition apparatus using an ALD method, two kinds of gases, i.e., ozone (O₃) as an oxidizer and a source gas which is obtained by vaporizing liquid containing a solvent and a hafnium precursor compound (e.g., a hafnium alkoxide or a hafnium amide such as tetrakis(dimethylamide)hafnium (TDMAH)) are used. Note that the chemical formula of tetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄. Examples of another material liquid include tetrakis(ethylmethylamide)hafnium.

For example, in the case where an aluminum oxide film is formed by a deposition apparatus using an ALD method, two kinds of gases, e.g., H₂O as an oxidizer and a source gas which is obtained by vaporizing liquid containing a solvent and an aluminum precursor compound (e.g., trimethylaluminum (TMA)) are used. Note that the chemical formula of trimethylaluminum is Al(CH₃)₃. Examples of another material liquid include tris(dimethylamide)aluminum, triisobutylaluminum, and aluminum tris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed by a deposition apparatus using an ALD method, hexachlorodisilane is adsorbed on a surface where a film is to be formed, chlorine included in the adsorbate is removed, and radicals of an oxidizing gas (e.g., 02 or dinitrogen monoxide) are supplied to react with the adsorbate.

For example, in the case where a tungsten film is formed with a deposition apparatus by an ALD method, a WF₆ gas and a B₂H₆ gas are sequentially introduced plural times to form an initial tungsten film, and then a tungsten film is formed using a WF₆ gas and an H₂ gas. Note that an SiH₄ gas may be used instead of a B₂H₆ gas.

For example, in the case where an oxide semiconductor film, e.g., an In—Ga—Zn—O film is formed using a deposition apparatus using an ALD method, an In(CH₃)₃ gas and an O₃ gas are sequentially introduced plural times to form an InO layer, a GaO layer is formed using a Ga(CH₃)₃ gas and an O₃ gas, and then a ZnO layer is formed using a Zn(CH₃)₂ gas and an O₃ gas. Note that the order of these layers is not limited to this example. A mixed compound layer such as an In—Ga—O layer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed by mixing of these gases. Note that although an H₂O gas which is obtained by bubbling with an inert gas such as Ar may be used instead of an O₃ gas, it is preferable to use an O₃ gas, which does not contain H. Instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used. Instead of a Ga(CH₃)₃ gas, a Ga(C₂H₅)₃ gas may be used.

Structure Example 2 of Semiconductor Device

A structure example different from that of the transistor 100 in FIGS. 1A to 1C is described with reference to FIGS. 2A to 2C. Note that in the case where a portion has a function similar to that described above, the same hatch pattern is applied to the portion, and the portion is not especially denoted by a reference numeral in some cases.

FIG. 2A is a top view of a transistor 160 that is a semiconductor device of one embodiment of the present invention. FIG. 2B is a cross-sectional view taken along dashed-dotted line X1-X2 illustrated in FIG. 2A, and FIG. 2C is a cross-sectional view taken along dashed-dotted line Y1-Y2 illustrated in FIG. 2A.

The transistor 160 is different from the transistor 100 described above in that the opening 142 b is not provided. The structure except for the opening 142 b is the same as the structure of the transistor 100, and the detailed description thereof is omitted.

As illustrated in FIGS. 2A and 2C, the opening 142 a may be provided for electrically connecting the conductive film 104 that functions as the first gate electrode to the conductive film 120 b that functions as the second gate electrode.

Structure Example 3 of Semiconductor Device

A structure example different from that of the transistor 100 in FIGS. 1A to 1C is described with reference to FIGS. 3A to 3C. Note that in the case where a portion has a function similar to that described above, the same hatch pattern is applied to the portion, and the portion is not especially denoted by a reference numeral in some cases.

FIG. 3A is a top view of a transistor 130 that is a semiconductor device of one embodiment of the present invention. FIG. 3B is a cross-sectional view taken along a dashed-dotted line X1-X2 in FIG. 3A. FIG. 3C is a cross-sectional view taken along a dashed-dotted line Y1-Y2 in FIG. 3A.

The transistor 130 illustrated in FIGS. 3A to 3C is different from the transistor 100 in that the conductive film 112 a that functions as one of a source and a drain concentrically surrounds the conductive film 112 b that functions as the other of the source and the drain when seen from the above.

FIG. 37A is a top view of the oxide semiconductor film 108 in the transistor 130. As illustrated in FIG. 37A, the oxide semiconductor film 108 includes a region 185 a that overlaps with the conductive film 112 a and a region 185 b that overlaps with the conductive film 112 b. The region 185 a and the region 185 b have edges opposed to each other. The length of the edge of the region 185 b is shorter than the length of the edge of the region 185 a when seen from the above.

An insulating film 121 and a conductive film 122 are provided so that the conductive film 112 b is electrically connected to an external terminal. The conductive film 122 is electrically connected to the conductive film 112 b through an opening 142 d. In addition, the conductive film 104 that functions as a first gate electrode is electrically connected to the conductive film 120 b that functions as a second gate electrode through the opening 142 a.

Although not illustrated, the conductive film 112 a may be electrically connected to a conductive film that functions as a pixel electrode.

Although not illustrated, the conductive film 122 may be electrically connected to a conductive film that functions as a pixel electrode.

For details of the conductive film 122, the description of the conductive films 104, 112 a, and 112 b may be referred to.

For the insulating film 121, one or more insulators selected from aluminum oxide, aluminum nitride oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, and the like can be used. Alternatively, for the insulating film 121, an organic resin such as a polyimide resin, a polyamide resin, an acrylic resin, a siloxane resin, an epoxy resin, or a phenol resin can be used.

The structure of the transistor 130 except for the above is the same as the structure of the transistor 100, and the detailed description thereof is omitted.

It is preferable that the conductive film 112 a function as a drain electrode and the conductive film 112 b function as a source electrode. The transistor 130 with a structure in which the drain electrode surrounds the source electrode is unlikely to be affected by the channel length modulation effect, and can show good saturation Vd-Id characteristics.

In FIG. 3A, L3 indicates the channel length of the transistor 130. As in the transistor 100 illustrated in FIGS. 1A to 1C, a channel length L3 of the transistor 130 refers to a distance between the edge of the conductive film 112 a and the edge of the conductive film 112 b when seen from above the oxide semiconductor film 108. In other words, the channel length L3 refers to a distance between an edge of the conductive film 112 a and an edge of the conductive film 112 b. The channel length L3 is greater than or equal to 1 μm and less than or equal to 100 μm, or greater than or equal to 1 μm and less than or equal to 30 μm.

In FIGS. 3A to 3C, W3 shown by a dashed-dotted line indicates the channel width of the transistor 130. The channel width W3 of the transistor 130 refers to the circumference of a concentric circle that is over the oxide semiconductor film 108 and equidistant from the edge of the conductive film 112 a and the edge of the conductive film 112 b.

Structure Example 4 of Semiconductor Device

A structure example different from that of the transistor 100 in FIGS. 1A to 1C is described with reference to FIGS. 4A to 4C. Note that in the case where a portion has a function similar to that described above, the same hatch pattern is applied to the portion, and the portion is not especially denoted by a reference numeral in some cases.

FIG. 4A is a top view of a transistor 140 that is a semiconductor device of one embodiment of the present invention. FIG. 4B is a cross-sectional view taken along a dashed-dotted line X1-X2 in FIG. 4A, and FIG. 4C is a cross-sectional view taken along a dashed-dotted line Y1-Y2 in FIG. 4A.

The transistor 140 illustrated in FIGS. 4A to 4C is different from the transistor 130 illustrated in FIGS. 3A to 3C in that the conductive film 112 a that functions as one of a source and a drain tetragonally surrounds the conductive film 112 b that functions as the other of the source and the drain when seen from the above. The structure except for the above is the same as the structure of the transistor 130, and the detailed description thereof is omitted.

FIG. 37B is a top view of the oxide semiconductor film 108 in the transistor 140. As illustrated in FIG. 37B, the oxide semiconductor film 108 includes the region 185 a that overlaps with the conductive film 112 a and the region 185 b that overlaps with the conductive film 112 b. The region 185 a and the region 185 b have edges opposed to each other. The length of the edge of the region 185 b is shorter than the length of the edge of the region 185 a when seen from the above.

It is preferable that the conductive film 112 a of the transistor 140 function as a drain electrode and the conductive film 112 b of the transistor 140 function as a source electrode as in the transistor 130. The transistor 140 with a structure in which the drain electrode surrounds the source electrode is unlikely to be affected by the channel length modulation effect, and can show good saturation Vd-Id characteristics.

In FIGS. 4A to 4C, L4 indicates the channel length of the transistor 140. The channel length L4 of the transistor 140 refers to a distance between the edge of the conductive film 112 a and the edge of the conductive film 112 b when seen from above the oxide semiconductor film 108. In other words, the channel length L4 refers to a distance between an edge of the conductive film 112 a and an edge of the conductive film 112 b. The channel length L4 is greater than or equal to 1 μm and less than or equal to 100 μm, or greater than or equal to 1 μm and less than or equal to 30 μm.

In FIG. 4A, W4 shown by a dashed-dotted line indicates the channel width of the transistor 140. The channel width W4 of the transistor 140 refers to the circumference of a tetragon that is over the oxide semiconductor film 108 and equidistant from the edge of the conductive film 112 a and the edge of the conductive film 112 b.

Structure Example 5 of Semiconductor Device

A structure example different from that of the transistor 100 in FIGS. 1A to 1C is described with reference to FIGS. 5A to 5C. Note that in the case where a portion has a function similar to that described above, the same hatch pattern is applied to the portion, and the portion is not especially denoted by a reference numeral in some cases.

FIG. 5A is a top view of a transistor 150 that is a semiconductor device of one embodiment of the present invention. FIG. 5B is a cross-sectional view taken along a dashed-dotted line X1-X2 in FIG. 5A, and FIG. 5C is a cross-sectional view taken along a dashed-dotted line Y1-Y2 in FIG. 5A.

The transistor 150 illustrated in FIGS. 5A to 5D is different from the transistor 100 illustrated in FIGS. 1A to 1C in that an edge of the conductive film 112 a that functions as one of a source and a drain and an edge of the conductive film 112 b that functions as the other of the source and the drain are opposed to each other and the oxide semiconductor film 108 forms a fan shape when seen from the above.

FIG. 5D is a top view of the oxide semiconductor film 108 in the transistor 150. As illustrated in FIG. 5D, the oxide semiconductor film 108 includes the region 185 a that overlaps with the conductive film 112 a and the region 185 b that overlaps with the conductive film 112 b. The region 185 a and the region 185 b have edges opposed to each other. The length of the edge of the region 185 a is shorter than the length of the edge of the region 185 b when seen from the above. The transistor 150 is different from the transistor 100 illustrated in FIGS. 1A to 1C in this point.

In FIGS. 5A, 5B, and 5D, L5 indicates the channel length of the transistor 150. As in the transistor 100 illustrated in FIGS. 1A to 1C, the channel length L5 of the transistor 150 refers to a distance between the edge of the conductive film 112 a and the edge of the conductive film 112 b when seen from above the oxide semiconductor film 108. The channel length L5 is greater than or equal to 1 μm and less than or equal to 100 μm, or greater than or equal to 1 μm and less than or equal to 30 μm.

The structure of the transistor 150 except for the above is the same as the structure of the transistor 100, and the detailed description thereof is omitted.

It is preferable that the conductive film 112 a of the transistor 150 function as a source electrode and the conductive film 112 b of the transistor 150 function as a drain electrode. The transistor 150 with the above structure is unlikely to be affected by the channel length modulation effect, and can show good saturation Vd-Id characteristics.

Structure Example 6 of Semiconductor Device

A structure example different from that of the transistor 100 in FIGS. 1A to 1C is described with reference to FIGS. 6A to 6C. Note that in the case where a portion has a function similar to that described above, the same hatch pattern is applied to the portion, and the portion is not especially denoted by a reference numeral in some cases.

FIG. 6A is a top view of a transistor 170 that is a semiconductor device of one embodiment of the present invention. FIG. 6B is a cross-sectional view taken along dashed-dotted line X1-X2 illustrated in FIG. 6A, and FIG. 6C is a cross-sectional view taken along dashed-dotted line Y1-Y2 illustrated in FIG. 6A.

The transistor 170 illustrated in FIGS. 6A to 6C is different from the transistor 150 illustrated in FIGS. 5A to 5D in that the shapes of the conductive films 120 b and 104 are bent with a curvature in accordance with the shapes of the conductive films 112 a and 112 b when seen from the above, and in that the opening 142 b illustrated in FIG. 5A is omitted.

The structure of the transistor 170 except for the above is the same as the structure of the transistor 150, and the detailed description thereof is omitted.

In FIGS. 6A and 6B, L7 indicates the channel length of the transistor 170. As in the transistor 100 illustrated in FIGS. 1A to 1C, the channel length L7 of the transistor 170 refers to a distance between the edge of the conductive film 112 a and the edge of the conductive film 112 b when seen from above the oxide semiconductor film 108. The channel length L7 is greater than or equal to 1 μm and less than or equal to 100 μm, or greater than or equal to 1 μm and less than or equal to 30 μm.

It is preferable that the conductive film 112 a of the transistor 170 function as a source electrode and the conductive film 112 b of the transistor 170 function as a drain electrode. The transistor 170 with the above structure is unlikely to be affected by the channel length modulation effect, and can show good saturation Vd-Id characteristics.

Structure Example 7 of Semiconductor Device

A structure example different from that of the transistor 100 in FIGS. 1A to 1C is described with reference to FIGS. 7A to 7C. Note that in the case where a portion has a function similar to that described above, the same hatch pattern is applied to the portion, and the portion is not especially denoted by a reference numeral in some cases.

FIG. 7A is a top view of a transistor 180 that is a semiconductor device of one embodiment of the present invention. FIG. 7B is a cross-sectional view taken along dashed-dotted line X1-X2 illustrated in FIG. 7A, and FIG. 7C is a cross-sectional view taken along dashed-dotted line Y1-Y2 illustrated in FIG. 7A.

The transistor 180 illustrated in FIGS. 7A to 7D is different from the transistor 100 illustrated in FIGS. 1A to 1C in that an edge of the conductive film 112 a that functions as one of a source and a drain and an edge of the conductive film 112 b that functions as the other of the source and the drain are opposed to each other and the oxide semiconductor film 108 forms a trapezoidal shape when seen from the above.

In FIGS. 7A to 7D, L8 indicates the channel length of the transistor 180. As in the transistor 100 illustrated in FIGS. 1A to 1C, a channel length L8 of the transistor 180 refers to a distance between the edge of the conductive film 112 a and the edge of the conductive film 112 b when seen from above the oxide semiconductor film 108. The channel length L8 is greater than or equal to 1 μm and less than or equal to 100 μm, or greater than or equal to 1 μm and less than or equal to 30 μm.

FIG. 7D is a top view of the oxide semiconductor film 108 in the transistor 180. As illustrated in FIG. 7D, the oxide semiconductor film 108 includes the region 185 a that overlaps with the conductive film 112 a and the region 185 b that overlaps with the conductive film 112 b. The region 185 a and the region 185 b have edges opposed to each other. The length of the edge of the region 185 a is shorter than the length of the edge of the region 185 b when seen from the above.

The structure of the transistor 180 except for the above is the same as the structure of the transistor 100, and the detailed description thereof is omitted.

It is preferable that the conductive film 112 a of the transistor 180 function as a source electrode and the conductive film 112 b of the transistor 150 function as a drain electrode. The transistor 180 with the above structure is unlikely to be affected by the channel length modulation effect, and can show good saturation Vd-Id characteristics.

Structure Example 8 of Semiconductor Device

Structure examples different from that of the transistor 100 in FIGS. 1A to 1C are described with reference to FIGS. 8A to 8D. Note that in the case where a portion has a function similar to that described above, the same hatch pattern is applied to the portion, and the portion is not especially denoted by a reference numeral in some cases.

FIGS. 8A and 8B each illustrate a cross-sectional view of a modification example of the transistor 100 in FIGS. 1B and 1C. FIGS. 8C and 8D each illustrate a cross-sectional view of another modification example of the transistor 100 in FIGS. 1B and 1C.

A transistor 100A in FIGS. 8A and 8B has the same structure as the transistor 100 in FIGS. 1B and 1C except that the oxide semiconductor film 108 has a three-layer structure. Specifically, the oxide semiconductor film 108 of the transistor 100A includes an oxide semiconductor film 108 a, an oxide semiconductor film 108 b, and an oxide semiconductor film 108 c.

A transistor 100B in FIGS. 8C and 8D has the same structure as the transistor 100 in FIGS. 1B and 1C except that the oxide semiconductor film 108 has a two-layer structure. Specifically, the oxide semiconductor film 108 of the transistor 100B includes the oxide semiconductor film 108 a and the oxide semiconductor film 108 b.

Here, a band structure including the oxide semiconductor films 108 a, 108 b, and 108 c and insulating films in contact with the oxide semiconductor films 108 b and 108 c is described with reference to FIGS. 9A and 9B.

FIG. 9A shows an example of a band structure in the thickness direction of a stack including the insulating film 107, the oxide semiconductor films 108 a, 108 b, and 108 c, and the insulating film 114. FIG. 9B shows an example of a band structure in the thickness direction of a stack including the insulating film 107, the oxide semiconductor films 108 b and 108 c, and the insulating film 114. For easy understanding, energy level of the conduction band minimum (Ec) of each of the insulating film 107, the oxide semiconductor films 108 a, 108 b, and 108 c, and the insulating film 114 is shown in the band structures.

In the band structure of FIG. 9A, a silicon oxide film is used as each of the insulating films 107 and 114, an oxide semiconductor film formed using a metal oxide target having an atomic ratio of metal elements of In:Ga:Zn=1:3:2 is used as the oxide semiconductor film 108 a, an oxide semiconductor film formed using a metal oxide target having an atomic ratio of metal elements of In:Ga:Zn=1:1:1 is used as the oxide semiconductor film 108 b, and an oxide semiconductor film formed using a metal oxide target having an atomic ratio of metal elements of In:Ga:Zn=1:3:2 is used as the oxide semiconductor film 108 c.

In the band structure of FIG. 9B, a silicon oxide film is used as each of the insulating films 107 and 114, an oxide semiconductor film formed using a metal oxide target having an atomic ratio of metal elements of In:Ga:Zn=1:1:1 is used as the oxide semiconductor film 108 b, and a metal oxide film formed using a metal oxide target having an atomic ratio of metal elements of In:Ga:Zn=1:3:2 is used as the oxide semiconductor film 108 c.

As illustrated in FIGS. 9A and 9B, the energy level of the conduction band minimum gradually varies between the oxide semiconductor film 108 a and the oxide semiconductor film 108 b and between the oxide semiconductor film 108 b and the oxide semiconductor film 108 c. In other words, the energy level of the conduction band minimum is continuously varied or continuously connected. To obtain such a band structure, there exists no impurity, which forms a defect state such as a trap center or a recombination center, at the interface between the oxide semiconductor film 108 a and the oxide semiconductor film 108 b or at the interface between the oxide semiconductor film 108 b and the oxide semiconductor film 108 c.

To form a continuous junction between the oxide semiconductor film 108 a and the oxide semiconductor film 108 b and between the oxide semiconductor film 108 b and the oxide semiconductor film 108 c, it is necessary to form the films successively without exposure to the air by using a multi-chamber deposition apparatus (sputtering apparatus) provided with a load lock chamber.

With the band structure of FIG. 9A or FIG. 9B, the oxide semiconductor film 108 b serves as a well, and a channel region is formed in the oxide semiconductor film 108 b in the transistor with the stacked-layer structure.

By providing the oxide semiconductor film 108 a and/or the oxide semiconductor film 108 c, the oxide semiconductor film 108 b can be distanced away from trap states.

In addition, the trap states might be more distant from the vacuum level than the energy level of the conduction band minimum (Ec) of the oxide semiconductor film 108 b functioning as a channel region, so that electrons are likely to be accumulated in the trap states. When the electrons are accumulated in the trap states, the electrons become negative fixed electric charge, so that the threshold voltage of the transistor is shifted in the positive direction. Therefore, it is preferable that the trap states be closer to the vacuum level than the energy level of the conduction band minimum (Ec) of the oxide semiconductor film 108 b. Such a structure inhibits accumulation of electrons in the trap states. As a result, the on-state current and the field-effect mobility of the transistor can be increased.

In FIGS. 9A and 9B, the energy level of the conduction band minimum of each of the oxide semiconductor films 108 a and 108 c is closer to the vacuum level than that of the oxide semiconductor film 108 b. Typically, a difference in energy level between the conduction band minimum of the oxide semiconductor film 108 b and the conduction band minimum of each of the oxide semiconductor films 108 a and 108 c is 0.15 eV or more or 0.5 eV or more and 2 eV or less or 1 eV or less. That is, the difference between the electron affinity of each of the oxide semiconductor films 108 a and 108 c and the electron affinity of the oxide semiconductor film 108 b is 0.15 eV or more or 0.5 eV or more and 2 eV or less or 1 eV or less.

In such a structure, the oxide semiconductor film 108 b serves as a main path of current and functions as a channel region. In addition, since the oxide semiconductor films 108 a and 108 c each include one or more metal elements included in the oxide semiconductor film 108 b in which a channel region is formed, interface scattering is less likely to occur at the interface between the oxide semiconductor film 108 a and the oxide semiconductor film 108 b or at the interface between the oxide semiconductor film 108 b and the oxide semiconductor film 108 c. Thus, the transistor can have high field-effect mobility because the movement of carriers is not hindered at the interface.

To prevent each of the oxide semiconductor films 108 a and 108 c from functioning as part of a channel region, a material having sufficiently low conductivity is used for the oxide semiconductor films 108 a and 108 c. Alternatively, a material which has a smaller electron affinity (a difference in energy level between the vacuum level and the conduction band minimum) than the oxide semiconductor film 108 b and has a difference in energy level in the conduction band minimum from the oxide semiconductor film 108 b (band offset) is used for the oxide semiconductor films 108 a and 108 c. Furthermore, to inhibit generation of a difference between threshold voltages due to the value of the drain voltage, it is preferable to form the oxide semiconductor films 108 a and 108 c using a material whose energy level of the conduction band minimum is closer to the vacuum level than that of the oxide semiconductor film 108 b by 0.2 eV or more, preferably 0.5 eV or more.

It is preferable that the oxide semiconductor films 108 a and 108 c not have a spinel crystal structure. This is because if the oxide semiconductor films 108 a and 108 c have a spinel crystal structure, constituent elements of the conductive films 112 a and 112 b might be diffused to the oxide semiconductor film 108 b at the interface between the spinel crystal structure and another region. Note that each of the oxide semiconductor film 108 a and 108 c is preferably a CAAC-OS, which is described later, in which case a higher blocking property against constituent elements of the conductive films 112 a and 112 b, for example, copper elements, is obtained.

The thickness of each of the oxide semiconductor films 108 a and 108 c is greater than or equal to a thickness that is capable of inhibiting diffusion of the constituent elements of the conductive films 112 a and 112 b to the oxide semiconductor film 108 b, and less than a thickness that inhibits supply of oxygen from the insulating film 114 to the oxide semiconductor film 108 b. For example, when the thickness of each of the oxide semiconductor films 108 a and 108 c is greater than or equal to 10 nm, diffusion of the constituent elements of the conductive films 112 a and 112 b to the oxide semiconductor film 108 b can be inhibited. When the thickness of each of the oxide semiconductor films 108 a and 108 c is less than or equal to 100 nm, oxygen can be effectively supplied from the insulating films 114 and 116 to the oxide semiconductor film 108 b.

When the oxide semiconductor films 108 a and 108 c are each an In-M-Zn oxide in which the atomic ratio of the element M (M is Ti, Ga, Y, Zr, La, Ce, Nd, or Hf) is higher than that of In, the energy gap of each of the oxide semiconductor films 108 a and 108 c can be large and the electron affinity thereof can be small. Therefore, a difference in electron affinity between the oxide semiconductor film 108 b and each of the oxide semiconductor films 108 a and 108 c may be controlled by the proportion of the element M. Furthermore, oxygen vacancy is less likely to be generated in the oxide semiconductor film in which the atomic ratio of Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf is higher than that of In because Ti, Ga, Y, Zr, La, Ce, Nd, and Hf each are a metal element that is strongly bonded to oxygen.

When an In-M-Zn oxide is used for the oxide semiconductor films 108 a and 108 c, the proportions of In and M, not taking Zn and O into consideration, is as follows: the atomic percentage of In is preferably less than 50 atomic % and the atomic percentage of M is greater than 50 atomic % and further preferably the atomic percentage of In is less than 25 atomic % and the atomic percentage of M is greater than 75 atomic %. Alternatively, a gallium oxide film may be used as each of the oxide semiconductor films 108 a and 108 c.

Furthermore, in the case where each of the oxide semiconductor films 108 a, 108 b, and 108 c is an In-M-Zn oxide, the proportion of M atoms in each of the oxide semiconductor films 108 a and 108 c is higher than that in the oxide semiconductor film 108 b. Typically, the proportion of M atoms in each of the oxide semiconductor films 108 a and 108 c is 1.5 or more times, preferably two or more times and further preferably three or more times as high as that in the oxide semiconductor film 108 b.

Furthermore, in the case where the oxide semiconductor films 108 a, 108 b, and 108 c are each an In-M-Zn oxide, when the oxide semiconductor film 108 b has an atomic ratio of In:M:Zn=x₁:y₁:z₁ and the oxide semiconductor films 108 a and 108 c each have an atomic ratio of In:M:Zn=x₂:y₂:z₂, y₂/x₂ is larger than y₁/x₁, preferably y₂/x₂ is 1.5 or more times as large as y₁/x₁, further preferably, y₂/x₂ is two or more times as large as y₁/x₁, and still further preferably y₂/x₂ is three or more times or four or more times as large as y₁/x₁. At this time, y₁ is preferably greater than or equal to x₁ in the oxide semiconductor film 108 b, because stable electrical characteristics of a transistor including the oxide semiconductor film 108 b can be achieved. However, when y₁ is three or more times as large as x₁, the field-effect mobility of the transistor including the oxide semiconductor film 108 b is reduced. Accordingly, y₁ is preferably smaller than three times x₁.

In the case where the oxide semiconductor film 108 b is an In-M-Zn oxide and a target having the atomic ratio of metal elements of In:M:Zn=x₁:y₁:z₁ is used for depositing the oxide semiconductor film 108 b, x₁/y₁ is preferably greater than or equal to ⅓ and less than or equal to 6 and further preferably greater than or equal to 1 and less than or equal to 6, and z₁/y₁ is preferably greater than or equal to ⅓ and less than or equal to 6 and further preferably greater than or equal to 1 and less than or equal to 6. Note that when z₁/y₁ is greater than or equal to 1 and less than or equal to 6, a CAAC-OS to be described later is easily formed as the oxide semiconductor film 108 b. Typical examples of the atomic ratio of the metal elements of the target are In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, and In:M:Zn=3:1:2.

In the case where the oxide semiconductor films 108 a and 108 c are each an In-M-Zn oxide and a target having an atomic ratio of metal elements of In:M:Zn=x₂:y₂:z₂ is used for depositing the oxide semiconductor films 108 a and 108 c, x ₂/y₂ is preferably less than x₁/y₁, and z₂/y₂ is preferably greater than or equal to ⅓ and less than or equal to 6 and further preferably greater than or equal to 1 and less than or equal to 6. When the atomic ratio of M with respect to indium is high, the energy gap of the oxide semiconductor films 108 a and 108 c can be large and the electron affinity thereof can be small; therefore, y₂/x₂ is preferably higher than or equal to 3 or higher than or equal to 4. Typical examples of the atomic ratio of the metal elements of the target include In:M:Zn=1:3:2, In:M:Zn=1:3:4, In:M:Zn=1:3:5, In:M:Zn=1:3:6, In:M:Zn=1:4:2, In:M:Zn=1:4:4, In:M:Zn=1:4:5, and In:M:Zn=1:5:5.

Furthermore, in the case where the oxide semiconductor films 108 a and 108 c are each an In-M oxide, when a divalent metal element (e.g., zinc) is not included as M, the oxide semiconductor films 108 a and 108 c which do not include a spinel crystal structure can be formed. As the oxide semiconductor films 108 a and 108 c, for example, an In—Ga oxide film can be used. The In—Ga oxide film can be formed by a sputtering method using an In—Ga metal oxide target (In:Ga=7:93), for example. To deposit the oxide semiconductor films 108 a and 108 c by a sputtering method using DC discharge, on the assumption that an atomic ratio of In:M is x:y, y/(x+y) is preferably less than or equal to 0.96 and further preferably less than or equal to 0.95, for example, 0.93.

In each of the oxide semiconductor films 108 a, 108 b, and 108 c, the proportions of the atoms in the above atomic ratio vary within a range of ±40% as an error.

The structures of the transistors of this embodiment can be freely combined with each other.

Method 1 for Manufacturing Semiconductor Device

Next, a method for manufacturing the transistor 100 that is a semiconductor device of one embodiment of the present invention is described in detail below with reference to FIGS. 10A to 10H, FIGS. 11A to 11F, and FIGS. 12A and 12H. FIGS. 10A to 10H, FIGS. 11A to 11F, and FIGS. 12A and 12H are cross-sectional views illustrating a method for manufacturing a semiconductor device. FIGS. 10A, 10C, 10E and 10G, FIGS. 11A, 11C, and 11E, and FIGS. 12A, 12C, 12E and 12G are cross-sectional views in the channel length direction of the transistor 100 in the middle of the manufacturing process. FIGS. 10B, 10D, 10F and 10H, FIGS. 11B, 11D, and 11F, and FIGS. 12B, 12D, 12F and 12H are cross-sectional views in the channel width direction of the transistor 100 in the middle of the manufacturing process.

Note that the films included in the transistor 100 (i.e., the insulating film, the oxide semiconductor film, the conductive film, and the like) can be formed by any of a sputtering method, a CVD method, a vacuum evaporation method, and a PLD method. Alternatively, a coating method or a printing method can be used. Although the sputtering method and a PECVD method are typical examples of the film formation method, a thermal CVD method may be used. As the thermal CVD method, an MOCVD method or an ALD method may be used, for example.

Deposition by the thermal CVD method may be performed in such a manner that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, and a source gas and an oxidizer are supplied to the chamber at a time and react with each other in the vicinity of the substrate or over the substrate. Thus, no plasma is generated in the deposition; therefore, the thermal CVD method has an advantage that no defect due to plasma damage is caused.

Deposition by the ALD method may be performed in such a manner that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, source gases for reaction are sequentially introduced into the chamber, and then the sequence of the gas introduction is repeated. For example, two or more kinds of source gases are sequentially supplied to the chamber by switching valves (also referred to as high-speed valves). In such a case, a first source gas is introduced, an inert gas (e.g., argon or nitrogen) or the like is introduced at the same time as or after introduction of the first gas so that the source gases are not mixed, and then a second source gas is introduced. Note that in the case where the first source gas and the inert gas are introduced at a time, the inert gas serves as a carrier gas, and the inert gas may also be introduced at the same time as the introduction of the second source gas. Alternatively, the first source gas may be exhausted by vacuum evacuation instead of the introduction of the inert gas, and then the second source gas may be introduced. The first source gas is adsorbed on the surface of the substrate to form a first single-atomic layer; then the second source gas is introduced to react with the first single-atomic layer; as a result, a second single-atomic layer is stacked over the first single-atomic layer, so that a thin film is formed.

The sequence of the gas introduction is repeated plural times until a desired thickness is obtained, whereby a thin film with excellent step coverage can be formed. The thickness of the thin film can be adjusted by the number of repetition times of the sequence of the gas introduction; therefore, an ALD method makes it possible to accurately adjust a thickness and thus is suitable for manufacturing a minute transistor.

First, a conductive film is formed over the substrate 102 and processed through a lithography process and an etching process, whereby the conductive film 104 that functions as the first gate electrode is formed. Then, the insulating films 106 and 107 which function as the first gate insulating film are formed over the conductive film 104 (see FIGS. 10A and 10B).

The conductive film 104 that functions as the first gate electrode can be formed by a sputtering method, a CVD method, a vacuum evaporation method, or a PLD method. Alternatively, a coating method or a printing method can be used. Although typical deposition methods are a sputtering method and PECVD method, a thermal CVD method, such as an MOCVD method, or an ALD method described above may be used.

In this embodiment, a glass substrate is used as the substrate 102, and as the conductive film 104 that functions as the first gate electrode, a 100-nm-thick tungsten film is formed by a sputtering method.

The insulating films 106 and 107 which function as the first gate insulating film can be formed by a sputtering method, a PECVD method, a thermal CVD method, a vacuum evaporation method, a PLD method, or the like. In this embodiment, a 400-nm-thick silicon nitride film as the insulating film 106 and a 50-nm-thick silicon oxynitride film as the insulating film 107 are formed by a PECVD method.

Note that the insulating film 106 can have a stacked-layer structure of silicon nitride films. Specifically, the insulating film 106 can have a three-layer structure of a first silicon nitride film, a second silicon nitride film, and a third silicon nitride film. An example of the three-layer structure is as follows.

For example, the first silicon nitride film can be formed to have a thickness of 50 nm under the conditions where silane at a flow rate of 200 sccm, nitrogen at a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 100 sccm are supplied as a source gas to a reaction chamber of a PECVD apparatus, the pressure in the reaction chamber is controlled to 100 Pa, and the power of 2000 W is supplied using a 27.12 MHz high-frequency power source.

The second silicon nitride film can be formed to have a thickness of 300 nm under the conditions where silane at a flow rate of 200 sccm, nitrogen at a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 2000 sccm are supplied as a source gas to the reaction chamber of the PECVD apparatus, the pressure in the reaction chamber is controlled to 100 Pa, and the power of 2000 W is supplied using a 27.12 MHz high-frequency power source.

The third silicon nitride film can be formed to have a thickness of 50 nm under the conditions where silane at a flow rate of 200 sccm and nitrogen at a flow rate of 5000 sccm are supplied as a source gas to the reaction chamber of the PECVD apparatus, the pressure in the reaction chamber is controlled to 100 Pa, and the power of 2000 W is supplied using a 27.12 MHz high-frequency power source.

Note that the first silicon nitride film, the second silicon nitride film, and the third silicon nitride film can each be formed at a substrate temperature of 350° C.

When the insulating film 106 has the three-layer structure of silicon nitride films, for example, in the case where a conductive film including Cu is used as the conductive film 104, the following effect can be obtained.

The first silicon nitride film can inhibit diffusion of a copper (Cu) element from the conductive film 104. The second silicon nitride film has a function of releasing hydrogen and can improve withstand voltage of the insulating film that functions as a gate insulating film. The third silicon nitride film releases a small amount of hydrogen and can inhibit diffusion of hydrogen released from the second silicon nitride film.

The insulating film 107 is preferably an insulating film including oxygen to improve characteristics of an interface with the oxide semiconductor film 108 formed later.

Next, the oxide semiconductor film 108 is formed over the insulating film 107 (see FIGS. 10C and 10D).

In this embodiment, an oxide semiconductor film is formed by a sputtering method using an In—Ga—Zn metal oxide target (having an atomic ratio of In:Ga:Zn=3:1:2), a mask is formed over the oxide semiconductor film through a lithography process, and the oxide semiconductor film is processed into a desired region, whereby the oxide semiconductor film 108 having an island shape is formed.

After the oxide semiconductor film 108 is formed, heat treatment may be performed at a temperature higher than or equal to 150° C. and lower than the strain point of the substrate, preferably higher than or equal to 200° C. and lower than or equal to 450° C., or further preferably higher than or equal to 300° C. and lower than or equal to 450° C. The heat treatment performed here serves as one kind of treatment for increasing the purity of the oxide semiconductor film and can reduce hydrogen, water, and the like included in the oxide semiconductor film 108. Note that the heat treatment for the purpose of reducing hydrogen, water, and the like may be performed before the oxide semiconductor film 108 is processed into an island shape.

An electric furnace, an RTA apparatus, or the like can be used for the heat treatment performed on the oxide semiconductor film 108. With the use of an RTA apparatus, the heat treatment can be performed at a temperature higher than or equal to the strain point of the substrate if the heating time is short. Therefore, the heat treatment time can be shortened.

Note that the heat treatment performed on the oxide semiconductor film 108 may be performed under an atmosphere of nitrogen, oxygen, ultra-dry air (air in which a water content is 20 ppm or less, preferably 1 ppm or less, or further preferably 10 ppb or less), or a rare gas (argon, helium, or the like). The atmosphere of nitrogen, oxygen, ultra-dry air, or a rare gas preferably does not contain hydrogen, water, and the like. Furthermore, after heat treatment performed under a nitrogen atmosphere or a rare gas atmosphere, heat treatment may be additionally performed in an oxygen atmosphere or an ultra-dry air atmosphere. As a result, hydrogen, water, and the like can be released from the oxide semiconductor film and oxygen can be supplied to the oxide semiconductor film at the same time. Consequently, the amount of oxygen vacancy in the oxide semiconductor film can be reduced.

In the case where the oxide semiconductor film 108 is formed by a sputtering method, as a sputtering gas, a rare gas (typically argon), oxygen, or a mixed gas of a rare gas and oxygen is used as appropriate. In the case of using the mixed gas of a rare gas and oxygen, the proportion of oxygen to a rare gas is preferably increased. In addition, increasing the purity of a sputtering gas is necessary. For example, as an oxygen gas or an argon gas used for a sputtering gas, a gas which is highly purified to have a dew point of −40° C. or lower, preferably −80° C. or lower, further preferably −100° C. or lower, or still further preferably −120° C. or lower is used, whereby entry of moisture and the like into the oxide semiconductor film 108 can be minimized.

In the case where the oxide semiconductor film 108 is formed by a sputtering method, a chamber in a sputtering apparatus is preferably evacuated to be a high vacuum state (to the degree of about 5×10⁻⁷ Pa to 1×10⁻⁴ Pa) with an adsorption vacuum evacuation pump such as a cryopump in order to remove water or the like, which serves as an impurity for the oxide semiconductor film 108, as much as possible. Alternatively, a turbo molecular pump and a cold trap are preferably combined so as to prevent a backflow of a gas, especially a gas including carbon or hydrogen, from an exhaust system to the inside of the chamber.

Next, the conductive films 112 a and 112 b which function as source and drain electrodes are formed over the insulating film 107 and the oxide semiconductor film 108 (see FIGS. 10E and 10F).

In this embodiment, the conductive films 112 a and 112 b are formed in the following manner: a stack formed of a 50-nm-thick tungsten film and a 400-nm-thick aluminum film is formed by a sputtering method, a mask is formed over the stack through a lithography process, and the stack is processed into desired regions. Although the conductive films 112 a and 112 b each have a two-layer structure in this embodiment, one embodiment of the present invention is not limited thereto. For example, the conductive films 112 a and 112 b each may have a three-layer structure of a 50-nm-thick tungsten film, a 400-nm-thick aluminum film, and a 100-nm-thick titanium film.

Next, a chemical solution 131 is applied onto the insulating film 107, the oxide semiconductor film 108, and the conductive films 112 a and 112 b for cleaning a surface of the oxide semiconductor film 108 (on the back channel side) (see FIGS. 10G and 10H).

The cleaning may be performed, for example, using a chemical solution such as phosphoric acid. The cleaning using a chemical solution such as a phosphoric acid can remove impurities (e.g., an element included in the conductive films 112 a and 112 b) attached to the surface of the oxide semiconductor film 108. Note that cleaning steps illustrated in FIGS. 10G and 10H are not necessarily performed, and in some cases, the cleaning does not needed.

Note that a recessed portion might be formed in part of the oxide semiconductor film 108 at the step of forming the conductive films 112 a and 112 b and/or the cleaning steps.

Next, over the insulating film 107, the oxide semiconductor film 108, and the conductive films 112 a and 112 b, the insulating films 114 and 116 are formed (see FIGS. 11A and 11B).

Note that after the insulating film 114 is formed, the insulating film 116 is preferably formed in succession without exposure to the air. After the insulating film 114 is formed, the insulating film 116 is formed in succession by adjusting at least one of the flow rate of a source gas, pressure, a high-frequency power, and a substrate temperature without exposure to the air, whereby the concentration of impurities attributed to the atmospheric component at the interface between the insulating film 114 and the insulating film 116 can be reduced, and oxygen in the insulating films 114 and 116 can be moved to the oxide semiconductor film 108; accordingly, the amount of oxygen vacancy in the oxide semiconductor film 108 can be reduced.

For example, as the insulating film 114, a silicon oxynitride film can be formed by a PECVD method. In this case, a deposition gas including silicon and an oxidizing gas are preferably used as a source gas. Typical examples of the deposition gas including silicon include silane, disilane, trisilane, and silane fluoride. Examples of the oxidizing gas include dinitrogen monoxide and nitrogen dioxide. An insulating film including nitrogen and having a small number of defects can be formed as the insulating film 114 by a PECVD method under the conditions where the ratio of the oxidizing gas to the deposition gas is higher than 20 times and lower than 100 times, or preferably higher than or equal to 40 times and lower than or equal to 80 times, and the pressure in a treatment chamber is lower than 100 Pa, or preferably lower than or equal to 50 Pa.

In this embodiment, a silicon oxynitride film is formed as the insulating film 114 by a PECVD method under the conditions where the substrate 102 is held at a temperature of 220° C., silane at a flow rate of 50 sccm and dinitrogen monoxide at a flow rate of 2000 sccm are used as a source gas, the pressure in the treatment chamber is 20 Pa, and a high-frequency power of 100 W at 13.56 MHz (1.6×10⁻² W/cm² as the power density) is supplied to parallel-plate electrodes.

As the insulating film 116, a silicon oxide film or a silicon oxynitride film is formed under the conditions where the substrate placed in a treatment chamber of the PECVD apparatus that is vacuum-evacuated is held at a temperature higher than or equal to 180° C. and lower than or equal to 280° C., or preferably higher than or equal to 200° C. and lower than or equal to 240° C., the pressure is greater than or equal to 100 Pa and less than or equal to 250 Pa, or preferably greater than or equal to 100 Pa and less than or equal to 200 Pa, with introduction of a source gas into the treatment chamber, and a high-frequency power greater than or equal to 0.17 W/cm² and less than or equal to 0.5 W/cm², or preferably greater than or equal to 0.25 W/cm² and less than or equal to 0.35 W/cm², is supplied to an electrode provided in the treatment chamber.

As the deposition conditions of the insulating film 116, the high-frequency power having the above power density is supplied to a reaction chamber having the above pressure, whereby the degradation efficiency of the source gas in plasma is increased, oxygen radicals are increased, and oxidation of the source gas is promoted; thus, the oxygen content in the insulating film 116 becomes higher than that in the stoichiometric composition. On the other hand, in the film formed at a substrate temperature within the above temperature range, the bond between silicon and oxygen is weak, and accordingly, part of oxygen in the film is released by heat treatment in a later step. Thus, an oxide insulating film which includes oxygen in excess of that in the stoichiometric composition and from which part of oxygen is released by heating can be formed.

Note that the insulating film 114 functions as a protection film for the oxide semiconductor film 108 in the step of forming the insulating film 116. Therefore, the insulating film 116 can be formed using the high-frequency power having a high power density while damage to the oxide semiconductor film 108 is reduced.

Note that in the deposition conditions of the insulating film 116, when the flow rate of the deposition gas including silicon with respect to the oxidizing gas is increased, the number of defects in the insulating film 116 can be reduced. Typically, it is possible to form an oxide insulating layer in which the number of defects is small, i.e., the spin density of a signal which appears at g=2.001 originating from a dangling bond of silicon is lower than 6×10¹⁷ spins/cm³, preferably lower than or equal to 3×10¹⁷ spins/cm³, or further preferably lower than or equal to 1.5×10¹⁷ spins/cm³, by ESR measurement. As a result, the reliability of the transistor can be improved.

Heat treatment may be performed after the insulating films 114 and 116 are formed. The heat treatment can reduce nitrogen oxide included in the insulating films 114 and 116. By the heat treatment, part of oxygen included in the insulating films 114 and 116 can be moved to the oxide semiconductor film 108, so that the amount of oxygen vacancy included in the oxide semiconductor film 108 can be reduced.

The temperature of the heat treatment performed on the insulating films 114 and 116 is typically higher than or equal to 150° C. and lower than or equal to 400° C., preferably higher than or equal to 300° C. and lower than or equal to 400° C., or further preferably higher than or equal to 320° C. and lower than or equal to 370° C. The heat treatment may be performed under an atmosphere of nitrogen, oxygen, ultra-dry air (air in which a water content is 20 ppm or less, preferably 1 ppm or less, or further preferably 10 ppb or less), or a rare gas (argon, helium, and the like). Note that an electric furnace, an RTA apparatus, and the like can be used for the heat treatment, in which it is preferable that hydrogen, water, and the like not be included in the nitrogen, oxygen, ultra-dry air, or rare gas.

In this embodiment, the heat treatment is performed at 350° C. under a nitrogen atmosphere for 1 hour.

Next, a protection film 117 is formed over the insulating film 116 (see FIGS. 11C and 11D).

The protection film 117 includes at least one of indium, zinc, titanium, aluminum, tungsten, tantalum, and molybdenum. For example, a conductive material such as an alloy including any of the metal elements, an alloy including any of the metal elements in combination, a metal oxide including any of the metal elements, a metal nitride including any of the metal elements, or a metal nitride oxide including any of the metal elements is used.

The protection film 117 can be formed using, for example, a tantalum nitride film, a titanium film, an indium tin oxide (hereinafter also referred to as ITO) film, an aluminum film, or an oxide semiconductor film (e.g., an IGZO film having an atomic ratio of In:Ga:Zn=1:4:5). The protection film 117 can be formed by a sputtering method. The thickness of the protection film 117 is preferably greater than or equal to 1 nm and less than or equal to 20 nm, or greater than or equal to 2 nm and less than or equal to 10 nm. In this embodiment, a 5-nm-thick indium tin oxide doped with silicon oxide (hereinafter referred to as ITSO) is used for the protection film 117.

Next, oxygen 133 is added to the insulating films 114 and 116 and the oxide semiconductor film 108 through the protection film 117 (see FIGS. 11E and 11F).

As a method for adding the oxygen 133 to the insulating films 114 and 116 and the oxide semiconductor film 108 through the protection film 117, an ion doping method, an ion implantation method, plasma treatment, or the like is given. By the bias application to the substrate side when the oxygen 133 is added, the oxygen 133 can be effectively added to the insulating films 114 and 116 and the oxide semiconductor film 108. As the bias, for example, power density can be greater than or equal to 1 W/cm² and less than or equal to 5 W/cm². When the protection film 117 is provided over the insulating film 116 and then oxygen is added, the protection film 117 functions as a protection film for inhibiting release of oxygen from the insulating film 116. Thus, a larger amount of oxygen can be added to the insulating films 114 and 116 and the oxide semiconductor film 108.

In the case where oxygen is introduced by plasma treatment, by making oxygen excited by a microwave to generate high density oxygen plasma, the amount of oxygen introduced into the insulating films 114 and 116 can be increased.

Next, the protection film 117 is removed (see FIGS. 12A and 12B).

A method for removing the protection film 117 is, for example, a dry etching method, a wet etching method, or a combination of a dry etching method and a wet etching method. In this embodiment, a wet etching method is employed for removing the protection film 117. Note that this embodiment shows an example in which the protection film 117 is removed, but one embodiment of the present invention is not limited to this. For example, the insulating film 118 may be formed over the protection film 117 without removing the protection film 117.

Next, the insulating film 118 is formed over the insulating film 116 (see FIGS. 12C and 12D).

Note that heat treatment may be performed before or after the formation of the insulating film 118, so that excess oxygen included in the insulating films 114 and 116 can be diffused to the oxide semiconductor film 108 to fill oxygen vacancy in the oxide semiconductor film 108. Alternatively, the insulating film 118 may be deposited by heating, so that excess oxygen included in the insulating films 114 and 116 can be diffused to the oxide semiconductor film 108 to fill oxygen vacancy in the oxide semiconductor film 108.

In the case where the insulating film 118 is formed by a PECVD method, the substrate temperature is preferably set to be higher than or equal to 300° C. and lower than or equal to 400° C., or further preferably higher than or equal to 320° C. and lower than or equal to 370° C., so that a dense film can be formed.

For example, in the case where a silicon nitride film is formed by a PECVD method as the insulating film 118, a deposition gas including silicon, nitrogen, and ammonia are preferably used as a source gas. A small amount of ammonia compared with the amount of nitrogen is used, whereby ammonia is dissociated in the plasma and activated species are generated. The activated species cleave a bond between silicon and hydrogen which are included in a deposition gas including silicon and a triple bond between nitrogen molecules. As a result, a dense silicon nitride film having few defects, in which bonds between silicon and nitrogen are promoted and bonds between silicon and hydrogen is few, can be formed. On the other hand, when the amount of ammonia with respect to nitrogen is large, decomposition of a deposition gas including silicon and decomposition of nitrogen are not promoted, so that a sparse silicon nitride film in which bonds between silicon and hydrogen remain and defects are increased is formed. Therefore, in the source gas, a flow rate ratio of the nitrogen to the ammonia is set to be greater than or equal to 5 and less than or equal to 50, or preferably greater than or equal to 10 and less than or equal to 50.

In this embodiment, with the use of a PECVD apparatus, a 50-nm-thick silicon nitride film is formed as the insulating film 118 using silane, nitrogen, and ammonia as a source gas. The flow rate of silane is 50 sccm, the flow rate of nitrogen is 5000 sccm, and the flow rate of ammonia is 100 sccm. The pressure in the treatment chamber is 100 Pa, the substrate temperature is 350° C., and high-frequency power of 1000 W is supplied to parallel-plate electrodes with a 27.12 MHz high-frequency power source. Note that the PECVD apparatus is a parallel-plate PECVD apparatus in which the electrode area is 6000 cm², and the power per unit area (power density) into which the supplied power is converted is 1.7×10⁻¹ W/cm².

Next, a mask is formed over the insulating film 118 through a lithography process, and the opening 142 c is formed in a desired region in the insulating films 114, 116, and 118. In addition, a mask is formed over the insulating film 118 through a lithography process, and the openings 142 a and 142 b are formed in desired regions in the insulating films 106, 107, 114, 116, and 118. Note that the opening 142 c reaches the conductive film 112 b. The openings 142 a and 142 b reach the conductive film 104 (see FIGS. 12E and 12F).

Note that the openings 142 a and 142 b and the opening 142 c may be formed in the same step or may be formed by different steps. In the case where the openings 142 a and 142 b and the opening 142 c are formed in the same step, for example, a gray-tone mask or a half-tone mask can be used. Moreover, the openings 142 a and 142 b may be formed in some steps. For example, the insulating films 106 and 107 are processed and then the insulating films 114, 116, and 118 are processed.

Next, a conductive film is formed over the insulating film 118 to cover the openings 142 a, 142 b, and 142 c, and the conductive film is processed into a desired shape, so that the conductive films 120 a and 120 b are formed (see FIGS. 12G and 12H).

For the conductive films 120 a and 120 b, for example, a material including one of indium (In), zinc (Zn), and tin (Sn) can be used. In particular, for the conductive films 120 a and 120 b, a light-transmitting conductive material such as indium oxide including tungsten oxide, indium zinc oxide including tungsten oxide, indium oxide including titanium oxide, indium tin oxide including titanium oxide, indium tin oxide (ITO), indium zinc oxide, or indium tin oxide to which silicon oxide is added (ITSO) can be used. The conductive films 120 a and 120 b can be formed by a sputtering method, for example. In this embodiment, a 110-nm-thick ITSO film is formed by a sputtering method.

Through the above process, the transistor 100 illustrated in FIGS. 1A to 1C can be fabricated.

The transistor 130 illustrated in FIGS. 3A to 3C and the transistor 140 illustrated in FIGS. 4A to 4C can be fabricated in such a manner that after the above-described steps, the insulating film 121 is formed, the opening 142 d is formed, and then the conductive film 122 is formed.

The insulating film 121 may be formed with, for example, an organic resin such as a polyimide resin, a polyamide resin, an acrylic resin, a siloxane resin, an epoxy resin, or a phenol resin. In this embodiment, a 1.5 μm-thick acrylic resin film is formed by a coating method.

For the conductive film 122, for example, a material including one of indium, zinc, and tin can be used. In this embodiment, a 110-nm-thick ITSO film is formed by a sputtering method.

The structure and method described in this embodiment can be implemented by being combined as appropriate with any of the other structures and methods described in the other embodiments.

Embodiment 2

In this embodiment, an oxide semiconductor film included in a semiconductor device of one embodiment of the present invention is described below in detail.

In this specification, the trigonal and rhombohedral crystal systems are included in the hexagonal crystal system.

Structure of Oxide Semiconductor

The structure of an oxide semiconductor is described below.

An oxide semiconductor is classified into a single crystal oxide semiconductor and a non-single-crystal oxide semiconductor. Examples of a non-single-crystal oxide semiconductor include a c-axis aligned crystalline oxide semiconductor (CAAC-OS), a polycrystalline oxide semiconductor, a nanocrystalline oxide semiconductor (nc-OS), an amorphous-like oxide semiconductor (a-like OS), and an amorphous oxide semiconductor.

From another perspective, an oxide semiconductor is classified into an amorphous oxide semiconductor and a crystalline oxide semiconductor. In addition, examples of a crystalline oxide semiconductor include a single crystal oxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor, and an nc-OS.

It is known that an amorphous structure is generally defined as being metastable and unfixed, and being isotropic and having no non-uniform structure. In other words, an amorphous structure has a flexible bond angle and a short-range order but does not have a long-range order.

This means that an inherently stable oxide semiconductor cannot be regarded as a completely amorphous oxide semiconductor. Moreover, an oxide semiconductor that is not isotropic (e.g., an oxide semiconductor film that has a periodic structure in a microscopic region) cannot be regarded as a completely amorphous oxide semiconductor. Note that an a-like OS has a periodic structure in a microscopic region, but at the same time has a void and has an unstable structure. For this reason, an a-like OS has physical properties similar to those of an amorphous oxide semiconductor.

CAAC-OS

First, a CAAC-OS is described.

A CAAC-OS is one of oxide semiconductors having a plurality of c-axis aligned crystal parts (also referred to as pellets).

In a combined analysis image (also referred to as a high-resolution transmission electron microscope (TEM) image of a bright-field image and a diffraction pattern of a CAAC-OS, which is obtained using a TEM, a plurality of pellets can be observed. However, in the high-resolution TEM image, a boundary between pellets, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS, a reduction in electron mobility due to the grain boundary is less likely to occur.

The CAAC-OS observed with a TEM is described below. FIG. 13A shows a high-resolution TEM image of a cross section of the CAAC-OS which is observed from a direction substantially parallel to the sample surface. The high-resolution TEM image is obtained with a spherical aberration corrector function. The high-resolution TEM image obtained with a spherical aberration corrector function is particularly referred to as a Cs-corrected high-resolution TEM image. The Cs-corrected high-resolution TEM image can be obtained with, for example, an atomic resolution analytical electron microscope JEM-ARM200F manufactured by JEOL Ltd.

FIG. 13B is an enlarged Cs-corrected high-resolution TEM image of a region (1) in FIG. 13A. FIG. 13B shows that metal atoms are arranged in a layered manner in a pellet. Each metal atom layer has a configuration reflecting unevenness of a surface over which a CAAC-OS film is formed (hereinafter, the surface is referred to as a formation surface) or a top surface of the CAAC-OS, and is arranged parallel to the formation surface or the top surface of the CAAC-OS.

As shown in FIG. 13B, the CAAC-OS has a characteristic atomic arrangement. The characteristic atomic arrangement is denoted by an auxiliary line in FIG. 13C. FIGS. 13B and 13C prove that the size of a pellet is greater than or equal to 1 nm or greater than or equal to 3 nm, and the size of a space caused by tilt of the pellets is approximately 0.8 nm. Therefore, the pellet can also be referred to as a nanocrystal (nc). Furthermore, a CAAC-OS can be referred to as an oxide semiconductor including c-axis aligned nanocrystals (CANC).

Here, according to the Cs-corrected high-resolution TEM images, the schematic arrangement of pellets 5100 of a CAAC-OS over a substrate 5120 is illustrated by such a structure in which bricks or blocks are stacked (see FIG. 13D). The part in which the pellets are tilted as observed in FIG. 13C corresponds to a region 5161 illustrated in FIG. 13D.

FIG. 14A shows a Cs-corrected high-resolution TEM image of a plane of the CAAC-OS observed from a direction substantially perpendicular to the sample surface. FIGS. 14B, 14C, and 14D are enlarged Cs-corrected high-resolution TEM images of regions (1), (2), and (3) in FIG. 14A, respectively. FIGS. 14B, 14C, and 14D indicate that metal atoms are arranged in a triangular, quadrangular, or hexagonal configuration in a pellet. However, there is no regularity of arrangement of metal atoms between different pellets.

Next, a CAAC-OS analyzed by X-ray diffraction (XRD) is described. For example, when the structure of a CAAC-OS including an InGaZnO₄ crystal is analyzed by an out-of-plane method, a peak appears at a diffraction angle (2θ) of around 31° as shown in FIG. 15A. This peak is derived from the (009) plane of the InGaZnO₄ crystal, which indicates that crystals in the CAAC-OS have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS.

Note that in structural analysis of the CAAC-OS by an out-of-plane method, another peak may appear when 2θ is around 36°, in addition to the peak at 28 of around 31°. The peak of 2θ at around 36° indicates that a crystal having no c-axis alignment is included in part of the CAAC-OS. It is preferable that in the CAAC-OS analyzed by an out-of-plane method, a peak appear when 2θ is around 31° and that a peak not appear when 2θ is around 36°.

On the other hand, in structural analysis of the CAAC-OS by an in-plane method in which an X-ray is incident on a sample in a direction substantially perpendicular to the c-axis, a peak appears when 2θ is around 56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal. In the case of the CAAC-OS, when analysis (φ scan) is performed with 2θ fixed at around 56° and with the sample rotated using a normal vector of the sample surface as an axis (φ axis), as shown in FIG. 15B, a peak is not clearly observed. In contrast, in the case of a single crystal oxide semiconductor of InGaZnO₄, when φ scan is performed with 2θ fixed at around 56°, as shown in FIG. 15C, six peaks which are derived from crystal planes equivalent to the (110) plane are observed. Accordingly, the structural analysis using XRD shows that the directions of a-axes and b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction is described. For example, when an electron beam with a probe diameter of 300 nm is incident on a CAAC-OS including an InGaZnO₄ crystal in a direction parallel to the sample surface, a diffraction pattern (also referred to as a selected-area transmission electron diffraction pattern) shown in FIG. 16A can be obtained. In this diffraction pattern, spots derived from the (009) plane of an InGaZnO₄ crystal are included. Thus, the electron diffraction also indicates that pellets included in the CAAC-OS have c-axis alignment and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS. Meanwhile, FIG. 16B shows a diffraction pattern obtained in such a manner that an electron beam with a probe diameter of 300 nm is incident on the same sample in a direction perpendicular to the sample surface. As shown in FIG. 16B, a ring-like diffraction pattern is observed. Thus, the electron diffraction also indicates that the a-axes and b-axes of the pellets included in the CAAC-OS do not have regular alignment. The first ring in FIG. 16B is considered to be derived from the (010) plane, the (100) plane, and the like of the InGaZnO₄ crystal. Furthermore, it is supposed that the second ring in FIG. 16B is derived from the (110) plane and the like.

As described above, the CAAC-OS is an oxide semiconductor with high crystallinity. Entry of impurities, formation of defects, or the like might decrease the crystallinity of an oxide semiconductor. This means that the CAAC-OS has small amounts of impurities and defects (e.g., oxygen vacancy).

Note that the impurity means an element other than the main components of the oxide semiconductor, such as hydrogen, carbon, silicon, or a transition metal element. For example, an element (specifically, silicon or the like) having higher strength of bonding to oxygen than a metal element included in an oxide semiconductor extracts oxygen from the oxide semiconductor, which results in disorder of the atomic arrangement and reduced crystallinity of the oxide semiconductor. A heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (or molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor and decreases crystallinity.

The characteristics of an oxide semiconductor having impurities or defects might be changed by light, heat, or the like. Impurities included in the oxide semiconductor might serve as carrier traps or carrier generation sources, for example. Furthermore, oxygen vacancy in the oxide semiconductor might serve as a carrier trap or serve as a carrier generation source when hydrogen is captured therein.

The CAAC-OS having small amounts of impurities and oxygen vacancy is an oxide semiconductor film with low carrier density. Such an oxide semiconductor is referred to as a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor. A CAAC-OS has a low impurity concentration and a low density of defect states. Thus, the CAAC-OS can be referred to as an oxide semiconductor having stable characteristics.

nc-OS

Next, an nc-OS is described.

An nc-OS has a region in which a crystal part is observed and a region in which a crystal part is not clearly observed in a high-resolution TEM image. In most cases, the size of a crystal part included in the nc-OS is greater than or equal to 1 nm and less than or equal to 10 nm, or greater than or equal to 1 nm and less than or equal to 3 nm. Note that an oxide semiconductor including a crystal part whose size is greater than 10 nm and less than or equal to 100 nm is sometimes referred to as a microcrystalline oxide semiconductor. In a high-resolution TEM image of the nc-OS, for example, a crystal grain boundary is not clearly observed in some cases. Note that there is a possibility that the origin of the nanocrystal is the same as that of a pellet in a CAAC-OS. Therefore, a crystal part of the nc-OS may be referred to as a pellet in the following description.

In the nc-OS, a microscopic region (e.g., a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic arrangement. There is no regularity of crystal orientation between different pellets in the nc-OS. Thus, the orientation of the whole film is not observed. Accordingly, the nc-OS cannot be distinguished from an a-like OS and an amorphous oxide semiconductor, depending on an analysis method. For example, when the nc-OS is analyzed by an out-of-plane method using an X-ray beam having a diameter larger than the size of a pellet, a peak which shows a crystal plane does not appear. Furthermore, a diffraction pattern like a halo pattern is observed when the nc-OS is subjected to electron diffraction using an electron beam with a probe diameter (e.g., 50 nm or larger) that is larger than the size of a pellet. Meanwhile, spots appear in a nanobeam electron diffraction pattern of the nc-OS when an electron beam having a probe diameter close to or smaller than the size of a pellet is used. Moreover, in a nanobeam electron diffraction pattern of the nc-OS, regions with high luminance in a circular (ring) pattern are shown in some cases. Also in a nanobeam electron diffraction pattern of the nc-OS layer, a plurality of spots is shown in a ring-like region in some cases.

Since there is no regularity of crystal orientation between the pellets (nanocrystals) as mentioned above, the nc-OS can also be referred to as an oxide semiconductor including random aligned nanocrystals (RANC) or an oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has high regularity as compared with an amorphous oxide semiconductor. Therefore, the nc-OS is likely to have a lower density of defect states than an a-like OS and an amorphous oxide semiconductor. Note that there is no regularity of crystal orientation between different pellets in the nc-OS. Therefore, the nc-OS has a higher density of defect states than the CAAC-OS.

a-Like OS

An a-like OS has a structure intermediate between those of the nc-OS and the amorphous oxide semiconductor.

In a high-resolution TEM image of the a-like OS, a void may be observed. Furthermore, in the high-resolution TEM image, there are a region where a crystal part is clearly observed and a region where a crystal part is not observed.

The a-like OS has an unstable structure because it contains a void. To verify that an a-like OS has an unstable structure as compared with a CAAC-OS and an nc-OS, a change in structure caused by electron irradiation is described below.

An a-like OS (sample A), an nc-OS (sample B), and a CAAC-OS (sample C) are prepared as samples subjected to electron irradiation. Each of the samples is an In—Ga—Zn oxide.

First, a high-resolution cross-sectional TEM image of each sample is obtained. The high-resolution cross-sectional TEM images show that all the samples have crystal parts.

Note that which part is regarded as a crystal part is determined as follows. It is known that a unit cell of the InGaZnO₄ crystal has a structure in which nine layers including three In—O layers and six Ga—Zn—O layers are stacked in the c-axis direction. The distance between the adjacent layers is equivalent to the lattice spacing on the (009) plane (also referred to as d value). The value is calculated to be 0.29 nm from crystal structural analysis. Accordingly, a portion where the lattice spacing between lattice fringes is greater than or equal to 0.28 nm and less than or equal to 0.30 nm is regarded as a crystal part of InGaZnO₄. Each of lattice fringes corresponds to the a-b plane of the InGaZnO₄ crystal.

FIG. 17 shows change in the average size of crystal parts (at 22 points to 45 points) in each sample. Note that the crystal part size corresponds to the length of a lattice fringe. FIG. 17 indicates that the crystal part size in the a-like OS increases with an increase in the cumulative electron dose. Specifically, as shown by (1) in FIG. 17, a crystal part of approximately 1.2 nm at the start of TEM observation (the crystal part is also referred to as an initial nucleus) grows to a size of approximately 2.6 nm at a cumulative electron dose of 4.2×10⁸ e⁻/nm². In contrast, the crystal part size in the nc-OS and the CAAC-OS shows little change from the start of electron irradiation to a cumulative electron dose of 4.2×10⁸ e⁻/nm². Specifically, as shown by (2) and (3) in FIG. 17, the average crystal sizes in an nc-OS and a CAAC-OS are approximately 1.4 nm and approximately 2.1 nm, respectively, regardless of the cumulative electron dose.

In this manner, growth of the crystal part in the a-like OS is induced by electron irradiation. In contrast, in the nc-OS and the CAAC-OS, growth of the crystal part is hardly induced by electron irradiation. Therefore, the a-like OS has an unstable structure as compared with the nc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS because it contains a void. Specifically, the density of the a-like OS is higher than or equal to 78.6% and lower than 92.3% of the density of the single crystal oxide semiconductor having the same composition. The density of each of the nc-OS and the CAAC-OS is higher than or equal to 92.3% and lower than 100% of the density of the single crystal oxide semiconductor having the same composition. Note that it is difficult to deposit an oxide semiconductor having a density of lower than 78% of the density of the single crystal oxide semiconductor.

For example, in the case of an oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with a rhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the case of the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of the a-like OS is higher than or equal to 5.0 g/cm³ and lower than 5.9 g/cm³. For example, in the case of the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, the density of each of the nc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lower than 6.3 g/cm³.

Note that there is a possibility that an oxide semiconductor having a certain composition cannot exist in a single crystal structure. In that case, single crystal oxide semiconductors with different compositions are combined at an adequate ratio, which makes it possible to calculate density equivalent to that of a single crystal oxide semiconductor with the desired composition. The density of a single crystal oxide semiconductor having the desired composition can be calculated using a weighted average according to the combination ratio of the single crystal oxide semiconductors with different compositions. Note that it is preferable to use as few kinds of single crystal oxide semiconductors as possible to calculate the density.

As described above, oxide semiconductors have various structures and various properties. Note that an oxide semiconductor may be a stacked layer including two or more of an amorphous oxide semiconductor, an a-like OS, an nc-OS, and a CAAC-OS, for example.

Deposition Model

An Example of a deposition model of a CAAC-OS is described below.

FIG. 18A is a schematic view of the inside of a deposition chamber where a CAAC-OS is deposited by a sputtering method.

A target 5130 is attached to a backing plate. A plurality of magnets is provided to face the target 5130 with the backing plate positioned therebetween. The plurality of magnets generates a magnetic field. A sputtering method in which the disposition rate is increased by utilizing a magnetic field of magnets is referred to as a magnetron sputtering method.

The substrate 5120 is placed to face the target 5130, and the distance d (also referred to as a target-substrate distance (T-S distance)) is greater than or equal to 0.01 m and less than or equal to 1 m, preferably greater than or equal to 0.02 m and less than or equal to 0.5 m. The deposition chamber is mostly filled with a deposition gas (e.g., an oxygen gas, an argon gas, or a mixed gas containing oxygen at 5 vol % or higher) and the pressure in the deposition chamber is controlled to be higher than or equal to 0.01 Pa and lower than or equal to 100 Pa, preferably higher than or equal to 0.1 Pa and lower than or equal to 10 Pa. Here, discharge starts by application of a voltage at a certain value or higher to the target 5130, and plasma is observed. The magnetic field forms a high-density plasma region in the vicinity of the target 5130. In the high-density plasma region, the deposition gas is ionized, so that an ion 5101 is generated. Examples of the ion 5101 include an oxygen cation (O⁺) and an argon cation (Ar⁺).

Here, the target 5130 has a polycrystalline structure which includes a plurality of crystal grains and in which a cleavage plane exists in at least one crystal grain. FIG. 19A shows a structure of an InGaZnO₄ crystal included in the target 5130 as an example. Note that FIG. 19A shows a structure of the case where the InGaZnO₄ crystal is observed from a direction parallel to the b-axis. FIG. 19A indicates that oxygen atoms in a Ga—Zn—O layer are positioned close to those in an adjacent Ga—Zn—O layer. The oxygen atoms have negative charge, whereby repulsive force is generated between the two adjacent Ga—Zn—O layers. As a result, the InGaZnO₄ crystal has a cleavage plane between the two adjacent Ga—Zn—O layers.

The ion 5101 generated in the high-density plasma region is accelerated toward the target 5130 side by an electric field, and then collides with the target 5130. At this time, a pellet 5100 a and a pellet 5100 b which are flat-plate-like (pellet-like) sputtered particles are separated and sputtered from the cleavage plane. Note that structures of the pellet 5100 a and the pellet 5100 b may be distorted by an impact of collision of the ion 5101.

The pellet 5100 a is a flat-plate-like (pellet-like) sputtered particle having a triangle plane, e.g., regular triangle plane. The pellet 5100 b is a flat-plate-like (pellet-like) sputtered particle having a hexagon plane, e.g., regular hexagon plane. Note that flat-plate-like (pellet-like) sputtered particles such as the pellet 5100 a and the pellet 5100 b are collectively called pellets 5100. The shape of a flat plane of the pellet 5100 is not limited to a triangle or a hexagon. For example, the flat plane may have a shape formed by combining two or more triangles. For example, a quadrangle (e.g., rhombus) may be formed by combining two triangles (e.g., regular triangles).

The thickness of the pellet 5100 is determined depending on the kind of deposition gas and the like. The thicknesses of the pellets 5100 are preferably uniform; the reason for this is described later. In addition, the sputtered particle preferably has a pellet shape with a small thickness as compared to a dice shape with a large thickness. For example, the thickness of the pellet 5100 is greater than or equal to 0.4 nm and less than or equal to 1 nm, preferably greater than or equal to 0.6 nm and less than or equal to 0.8 nm. In addition, for example, the width of the pellet 5100 is greater than or equal to 1 nm and less than or equal to 3 nm, preferably greater than or equal to 1.2 nm and less than or equal to 2.5 nm. The pellet 5100 corresponds to the initial nucleus in the description of (1) in FIG. 17. For example, when the ion 5101 collides with the target 5130 including an In—Ga—Zn oxide, the pellet 5100 that includes three layers of a Ga—Zn—O layer, an In—O layer, and a Ga—Zn—O layer as shown in FIG. 19B is separated. Note that FIG. 19C shows the structure of the separated pellet 5100 which is observed from a direction parallel to the c-axis. The pellet 5100 has a nanometer-sized sandwich structure including two Ga—Zn—O layers (pieces of bread) and an In—O layer (filling).

The pellet 5100 may receive a charge when passing through the plasma, so that side surfaces thereof are negatively or positively charged. In the pellet 5100, for example, an oxygen atom positioned on its side surface may be negatively charged. When the side surfaces are charged with the same polarity, charges repel each other, and accordingly, the pellet 5100 can maintain a flat-plate (pellet) shape. In the case where a CAAC-OS is an In—Ga—Zn oxide, there is a possibility that an oxygen atom bonded to an indium atom is negatively charged. There is another possibility that an oxygen atom bonded to an indium atom, a gallium atom, or a zinc atom is negatively charged. In addition, the pellet 5100 may grow by being bonded with an indium atom, a gallium atom, a zinc atom, an oxygen atom, or the like when passing through plasma. A difference in size between (2) and (1) in FIG. 17 corresponds to the amount of growth in plasma. Here, in the case where the temperature of the substrate 5120 is at around room temperature, the pellet 5100 on the substrate 5120 hardly grows; thus, an nc-OS is formed (see FIG. 18B). An nc-OS can be deposited when the substrate 5120 has a large size because the deposition of an nc-OS can be carried out at room temperature. Note that in order that the pellet 5100 grows in plasma, it is effective to increase deposition power in sputtering. High deposition power can stabilize the structure of the pellet 5100.

As shown in FIGS. 18A and 18B, the pellet 5100 flies like a kite in plasma and flutters up to the substrate 5120. Since the pellets 5100 are charged, when the pellet 5100 gets close to a region where another pellet 5100 has already been deposited, repulsion is generated. Here, above the substrate 5120, a magnetic field in a direction parallel to the top surface of the substrate 5120 (also referred to as a horizontal magnetic field) is generated. A potential difference is given between the substrate 5120 and the target 5130, and accordingly, current flows from the substrate 5120 toward the target 5130. Thus, the pellet 5100 is given a force (Lorentz force) on the top surface of the substrate 5120 by an effect of the magnetic field and the current. This is explainable with Fleming's left-hand rule.

The mass of the pellet 5100 is larger than that of an atom. Therefore, to move the pellet 5100 over the top surface of the substrate 5120, it is important to apply some force to the pellet 5100 from the outside. One kind of the force may be force which is generated by the action of a magnetic field and current. In order to apply a sufficient force to the pellet 5100 so that the pellet 5100 moves over a top surface of the substrate 5120, it is preferable to provide, on the top surface, a region where the magnetic field in a direction parallel to the top surface of the substrate 5120 is 10 G or higher, preferably 20 G or higher, further preferably 30 G or higher, still further preferably 50 G or higher. Alternatively, it is preferable to provide, on the top surface, a region where the magnetic field in a direction parallel to the top surface of the substrate 5120 is 1.5 times or higher, preferably twice or higher, further preferably 3 times or higher, still further preferably 5 times or higher as high as the magnetic field in a direction perpendicular to the top surface of the substrate 5120.

At this time, the magnets and the substrate 5120 are moved or rotated relatively, whereby the direction of the horizontal magnetic field on the top surface of the substrate 5120 continues to change. Therefore, the pellet 5100 can be moved in various directions on the top surface of the substrate 5120 by receiving forces in various directions.

Furthermore, as shown in FIG. 18A, when the substrate 5120 is heated, resistance between the pellet 5100 and the substrate 5120 due to friction or the like is low. As a result, the pellet 5100 glides above the top surface of the substrate 5120. The glide of the pellet 5100 is caused in a state where its flat plane faces the substrate 5120. Then, when the pellet 5100 reaches the side surface of another pellet 5100 that has been already deposited, the side surfaces of the pellets 5100 are bonded. At this time, the oxygen atom on the side surface of the pellet 5100 is released. With the released oxygen atom, oxygen vacancies in a CAAC-OS might be filled; thus, the CAAC-OS has a low density of defect states. Note that the temperature of the top surface of the substrate 5120 is, for example, higher than or equal to 100° C. and lower than 500° C., higher than or equal to 150° C. and lower than 450° C., or higher than or equal to 170° C. and lower than 400° C. Hence, even when the substrate 5120 has a large size, it is possible to deposit a CAAC-OS.

Furthermore, the pellet 5100 is heated on the substrate 5120, whereby atoms are rearranged, and the structure distortion caused by the collision of the ion 5101 can be reduced. The pellet 5100 whose structure distortion is reduced is substantially single crystal. Even when the pellets 5100 are heated after being bonded, expansion and contraction of the pellet 5100 itself hardly occur, which is caused by turning the pellet 5100 into substantially single crystal. Thus, formation of defects such as a grain boundary due to expansion of a space between the pellets 5100 can be prevented, and accordingly, generation of crevasses can be prevented.

The CAAC-OS does not have a structure like a board of a single crystal oxide semiconductor but has arrangement with a group of pellets 5100 (nanocrystals) like stacked bricks or blocks. Furthermore, a grain boundary does not exist between the pellets 5100. Therefore, even when deformation such as shrink occurs in the CAAC-OS owing to heating during deposition, or heating or bending after deposition, it is possible to relieve local stress or release distortion. Therefore, this structure is suitable for a flexible semiconductor device. Note that the nc-OS has arrangement in which pellets 5100 (nanocrystals) are randomly stacked.

When the target 5130 is sputtered with the ion 5101, in addition to the pellets 5100, zinc oxide or the like may be separated. The zinc oxide is lighter than the pellet 5100 and thus reaches the top surface of the substrate 5120 before the pellet 5100. As a result, the zinc oxide forms a zinc oxide layer 5102 with a thickness greater than or equal to 0.1 nm and less than or equal to 10 nm, greater than or equal to 0.2 nm and less than or equal to 5 nm, or greater than or equal to 0.5 nm and less than or equal to 2 nm. FIGS. 20A to 20D are cross-sectional schematic views.

As illustrated in FIG. 20A, a pellet 5105 a and a pellet 5105 b are deposited over the zinc oxide layer 5102. Here, side surfaces of the pellet 5105 a and the pellet 5105 b are in contact with each other. In addition, a pellet 5105 c is deposited over the pellet 5105 b, and then glides over the pellet 5105 b. Furthermore, a plurality of particles 5103 separated from the target together with the zinc oxide is crystallized by heat from the substrate 5120 to form a region 5105 a 1 on another side surface of the pellet 5105 a. Note that the plurality of particles 5103 may contain oxygen, zinc, indium, gallium, or the like.

Then, as illustrated in FIG. 20B, the region 5105 a 1 grows to part of the pellet 5105 a to form a pellet 5105 a 2. In addition, a side surface of the pellet 5105 c is in contact with another side surface of the pellet 5105 b.

Next, as illustrated in FIG. 20C, a pellet 5105 d is deposited over the pellet 5105 a 2 and the pellet 5105 b, and then glides over the pellet 5105 a 2 and the pellet 5105 b. Furthermore, a pellet 5105 e glides toward another side surface of the pellet 5105 c over the zinc oxide layer 5102.

Then, as illustrated in FIG. 20D, the pellet 5105 d is placed so that a side surface of the pellet 5105 d is in contact with a side surface of the pellet 5105 a 2. Furthermore, a side surface of the pellet 5105 e is in contact with another side surface of the pellet 5105 c. A plurality of particles 5103 separated from the target 5130 together with the zinc oxide is crystallized by heat from the substrate 5120 to form a region 5105 d 1 on another side surface of the pellet 5105 d.

As described above, deposited pellets are placed to be in contact with each other and then growth is caused at side surfaces of the pellets, whereby a CAAC-OS is formed over the substrate 5120. Therefore, each pellet of the CAAC-OS is larger than that of the nc-OS. A difference in size between (3) and (2) in FIG. 17 corresponds to the amount of growth after deposition.

When spaces between pellets are extremely small, the pellets may form a large pellet. The large pellet has a single crystal structure. For example, the size of the pellet may be greater than or equal to 10 nm and less than or equal to 200 nm, greater than or equal to 15 nm and less than or equal to 100 nm, or greater than or equal to 20 nm and less than or equal to 50 nm, when seen from the above. In this case, in an oxide semiconductor used for a minute transistor, a channel formation region might be fit inside the large pellet. That is, the region having a single crystal structure can be used as the channel formation region. Furthermore, when the size of the pellet is increased, the region having a single crystal structure can be used as the channel formation region, the source region, and the drain region of the transistor.

In this manner, when the channel formation region or the like of the transistor is formed in a region having a single crystal structure, the frequency characteristics of the transistor can be increased in some cases.

As shown in such a model, the pellets 5100 are considered to be deposited on the substrate 5120. Thus, a CAAC-OS can be deposited even when a formation surface does not have a crystal structure; therefore, a growth mechanism in this case is different from epitaxial growth. In addition, laser crystallization is not needed for formation of a CAAC-OS, and a uniform film can be formed even over a large-sized glass substrate or the like. For example, even when the top surface (formation surface) of the substrate 5120 has an amorphous structure (e.g., the top surface is formed of amorphous silicon oxide), a CAAC-OS can be formed.

In addition, it is found that in formation of the CAAC-OS, the pellets 5100 are arranged in accordance with the top surface shape of the substrate 5120 that is the formation surface even when the formation surface has unevenness. For example, in the case where the top surface of the substrate 5120 is flat at the atomic level, the pellets 5100 are arranged so that flat planes parallel to the a-b plane face downwards. In the case where the thicknesses of the pellets 5100 are uniform, a layer with a uniform thickness, flatness, and high crystallinity is formed. By stacking n layers (n is a natural number), the CAAC-OS can be obtained.

In the case where the top surface of the substrate 5120 has unevenness, a CAAC-OS in which n layers (n is a natural number) in each of which the pellets 5100 are arranged along the unevenness are stacked is formed. Since the substrate 5120 has unevenness, a space is easily generated between the pellets 5100 in the CAAC-OS in some cases. Note that, even in such a case, owing to intermolecular force, the pellets 5100 are arranged so that a space between the pellets is as small as possible even on the unevenness surface. Therefore, even when the formation surface has unevenness, a CAAC-OS with high crystallinity can be obtained.

Since a CAAC-OS is deposited in accordance with such a model, the sputtered particle preferably has a pellet shape with a small thickness. Note that when the sputtered particles have a dice shape with a large thickness, planes facing the substrate 5120 vary; thus, the thicknesses and orientations of the crystals cannot be uniform in some cases.

According to the deposition model described above, a CAAC-OS with high crystallinity can be formed even on a formation surface with an amorphous structure.

Embodiment 3

In this embodiment, an example of a display device that includes any of the transistors described in the embodiment above is described below with reference to FIG. 21, FIG. 22, FIG. 23, FIGS. 24A and 24B, and FIGS. 25A and 25B.

FIG. 21 is a top view of an example of a display device. A display device 700 illustrated in FIG. 21 includes a pixel portion 702 provided over a first substrate 701; a source driver circuit portion 704 and a gate driver circuit portion 706 provided over the first substrate 701; a sealant 712 provided to surround the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706; and a second substrate 705 provided to face the first substrate 701. The first substrate 701 and the second substrate 705 are sealed with the sealant 712. That is, the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706 are sealed with the first substrate 701, the sealant 712, and the second substrate 705. Although not illustrated in FIG. 21, a display element is provided between the first substrate 701 and the second substrate 705.

In the display device 700, a flexible printed circuit (FPC) terminal portion 708 electrically connected each other to the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706 is provided in a region different from the region which is surrounded by the sealant 712 and positioned over the first substrate 701. Furthermore, an FPC 716 is connected to the FPC terminal portion 708, and a variety of signals and the like are supplied to the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706 through the FPC 716. Furthermore, a signal line 710 is connected to the pixel portion 702, the source driver circuit portion 704, the gate driver circuit portion 706, and the FPC terminal portion 708. Various signals and the like are applied to the pixel portion 702, the source driver circuit portion 704, the gate driver circuit portion 706, and the FPC terminal portion 708 via the signal line 710 from the FPC 716.

A plurality of gate driver circuit portions 706 may be provided in the display device 700. An example of the display device 700 in which the source driver circuit portion 704 and the gate driver circuit portion 706 are formed over the first substrate 701 where the pixel portion 702 is also formed is described; however, the structure is not limited thereto. For example, only the gate driver circuit portion 706 may be formed over the first substrate 701 or only the source driver circuit portion 704 may be formed over the first substrate 701. In this case, a substrate where a source driver circuit, a gate driver circuit, or the like is formed (e.g., a driver-circuit substrate formed using a single-crystal semiconductor film or a polycrystalline semiconductor film) may be mounted on the first substrate 701. Note that there is no particular limitation on the method for connecting a separately prepared driver circuit substrate, and a chip on glass (COG) method, a wire bonding method, or the like can be used.

The pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706 included in the display device 700 include a plurality of transistors. As the plurality of transistors, any of the transistors that are the semiconductor devices of embodiments of the present invention can be used.

The display device 700 can include any of various display elements. Examples of the display elements are a liquid crystal element, an electroluminescent (EL) element (e.g., an EL element including organic and inorganic materials, an organic EL element, or an inorganic EL element), an LED (e.g., a white LED, a red LED, a green LED, or a blue LED), a transistor (a transistor that emits light depending on current), an electron emitter, electronic ink, an electrophoretic element, a grating light valve (GLV), a plasma display panel (PDP), a display element using micro electro mechanical system (MEMS), a digital micromirror device (DMD), a digital micro shutter (DMS), MIRASOL (registered trademark), an interferometric modulator display (IMOD) element, a MEMS shutter display element, an optical-interference-type MEMS display element, an electrowetting element, a piezoelectric ceramic display, and a display element using a carbon nanotube. Other than the above, a display medium whose contrast, luminance, reflectance, transmittance, or the like is changed by electric or magnetic action may be included in the display device 700. Examples of display devices having EL elements include an EL display. Examples of display devices including electron emitters include a field emission display (FED) and an SED-type flat panel display (SED: surface-conduction electron-emitter display). Examples of display devices including liquid crystal elements include a liquid crystal display (e.g., a transmissive liquid crystal display, a transflective liquid crystal display, a reflective liquid crystal display, a direct-view liquid crystal display, or a projection liquid crystal display). An example of a display device including electronic ink or electrophoretic elements is electronic paper. In the case of a transflective liquid crystal display or a reflective liquid crystal display, some of or all of pixel electrodes function as reflective electrodes. For example, some or all of pixel electrodes are formed to include aluminum, silver, or the like. In such a case, a memory circuit such as an SRAM can be provided under the reflective electrodes, leading to lower power consumption.

As a display method in the display device 700, a progressive method, an interlace method, or the like can be employed. Furthermore, color elements controlled in a pixel at the time of color display are not limited to three colors: R, G, and B (R, G, and B correspond to red, green, and blue, respectively). For example, four pixels of the R pixel, the G pixel, the B pixel, and a W (white) pixel may be included. Alternatively, a color element may be composed of two colors among R, G, and B as in PenTile layout. The two colors may differ among color elements. Alternatively, one or more colors of yellow, cyan, magenta, and the like may be added to RGB. Furthermore, the size of a display region may be different depending on respective dots of the color components. Embodiments of the disclosed invention are not limited to a display device for color display; the disclosed invention can also be applied to a display device for monochrome display.

A coloring layer (also referred to as a color filter) may be used in order to obtain a full-color display device in which white light (W) for a backlight (e.g., an organic EL element, an inorganic EL element, an LED, or a fluorescent lamp) is used. As the coloring layer, red (R), green (G), blue (B), yellow (Y), or the like may be combined as appropriate, for example. With the use of the coloring layer, higher color reproducibility can be obtained than in the case without the coloring layer. In this case, by providing a region with the coloring layer and a region without the coloring layer, white light in the region without the coloring layer may be directly utilized for display. By partly providing the region without the coloring layer, a decrease in luminance due to the coloring layer can be suppressed, and 20% to 30% of power consumption can be reduced in some cases when an image is displayed brightly. Note that in the case where full-color display is performed using a self-luminous element such as an organic EL element or an inorganic EL element, elements may emit light of their respective colors R, G, B, Y, and W. By using a self-luminous element, power consumption can be further reduced as compared with the case of using the coloring layer in some cases.

In this embodiment, a structure including a liquid crystal element or an EL element as a display element is described with reference to FIG. 22 and FIG. 23. Note that FIG. 22 is a cross-sectional view taken along the dashed line Q-R in FIG. 21, and shows a structure including a liquid crystal element as a display element. FIG. 23 is a cross-sectional view taken along the dashed line Q-R in FIG. 21, and shows a structure including an EL element as a display element.

Common portions between FIG. 22 and FIG. 23 are described first, and then different portions are described.

Common Portions in Display Devices

The display device 700 illustrated in FIG. 22 and FIG. 23 include a lead wiring portion 711, the pixel portion 702, the source driver circuit portion 704, and the FPC terminal portion 708. Note that the lead wiring portion 711 includes the signal line 710. The pixel portion 702 includes a transistor 750 and a capacitor 790. The source driver circuit portion 704 includes a transistor 752.

Any of the transistors described above can be used as the transistors 750 and 752.

The transistors used in this embodiment each include an oxide semiconductor film which is highly purified and in which formation of oxygen vacancy is suppressed. In the transistor, the current in an off state (off-state current) can be made small. Accordingly, an electrical signal such as an image signal can be held for a longer period, and a writing interval can be set longer in an on state. Accordingly, frequency of refresh operation can be reduced, which leads to an effect of suppressing power consumption.

In addition, the transistor used in this embodiment can have relatively high field-effect mobility and thus is capable of high speed operation. For example, with such a transistor which can operate at high speed used for a liquid crystal display device, a switching transistor in a pixel portion and a driver transistor in a driver circuit portion can be formed over one substrate. That is, a semiconductor device formed using a silicon wafer or the like is not additionally needed as a driver circuit, whereby the number of components of the semiconductor device can be reduced. In addition, the transistor which can operate at high speed can be used also in the pixel portion, whereby a high-quality image can be provided.

The capacitor 790 includes a dielectric between a pair of electrodes. Specifically, a conductive film which is formed using the same step as a conductive film that functions as a gate electrode of the transistor 750 is used as one electrode of the capacitor 790, and a conductive film that functions as a source electrode or a drain electrode of the transistor 750 is used as the other electrode of the capacitor 790. Furthermore, an insulating film that functions as a gate insulating film of the transistor 750 is used as the dielectric between the pair of electrodes.

In FIG. 22 and FIG. 23, insulating films 764, 766, and 768, a protection film 767, and a planarization insulating film 770 are formed over the transistor 750, the transistor 752, and the capacitor 790.

The insulating films 764, 766, and 768 can be formed using materials and methods similar to those of the insulating films 114, 116, and 118 described in the above embodiment, respectively. The protection film 767 can be formed using a material and a method similar to those of the protection film 117 described in the above embodiment. The planarization insulating film 770 can be formed using a heat-resistant organic material, such as a polyimide resin, an acrylic resin, a polyimide amide resin, a benzocyclobutene resin, a polyamide resin, or an epoxy resin. Note that the planarization insulating film 770 may be formed by stacking a plurality of insulating films formed from these materials. Alternatively, a structure without the planarization insulating film 770 may be employed.

The signal line 710 is formed in the same steps as conductive films which function as source and drain electrodes of the transistor 750 or 752. Note that the signal line 710 may be formed using a conductive film which is formed in different steps as a source electrode and a drain electrode of the transistor 750 or 752, for example, a conductive film that functions as a gate electrode may be used. In the case where the signal line 710 is formed using a material including a copper element, signal delay or the like due to wiring resistance is reduced, which enables display on a large screen.

The FPC terminal portion 708 includes a connection electrode 760, an anisotropic conductive film 780, and the FPC 716. Note that the connection electrode 760 is formed in the same steps as conductive films which function as source and drain electrodes of the transistor 750 or 752. The connection electrode 760 is electrically connected to a terminal included in the FPC 716 through the anisotropic conductive film 780.

For example, a glass substrate can be used as the first substrate 701 and the second substrate 705. A flexible substrate may be used as the first substrate 701 and the second substrate 705. Examples of the flexible substrate include a plastic substrate.

A structure body 778 is provided between the first substrate 701 and the second substrate 705. The structure body 778 is a columnar spacer obtained by selective etching of an insulating film and provided to control the distance (cell gap) between the first substrate 701 and the second substrate 705. Note that a spherical spacer may be used as the structure body 778. Although the structure in which the structure body 778 is provided on the first substrate 701 side is described as an example in this embodiment, one embodiment of the present invention is not limited thereto. For example, a structure in which the structure body 778 is provided on the second substrate 705 side, or a structure in which both of the first substrate 701 and the second substrate 705 are provided with the structure body 778 may be employed.

Furthermore, a light-blocking film 738 that functions as a black matrix, a coloring film 736 that functions as a color filter, and an insulating film 734 in contact with the light-blocking film 738 and the coloring film 736 are provided on the second substrate 705 side.

Structure Example of Display Device Using Liquid Crystal Element as Display Element

The display device 700 in FIG. 22 includes a liquid crystal element 775. The liquid crystal element 775 includes a conductive film 772, a conductive film 774, and a liquid crystal layer 776. The conductive film 774 is provided on the second substrate 705 side and functions as a counter electrode. The display device 700 in FIG. 22 is capable of displaying an image in such a manner that transmission or non-transmission is controlled by change in the alignment state of the liquid crystal layer 776 depending on a voltage applied to the conductive film 772 and the conductive film 774.

The conductive film 772 is connected to the conductive film that functions as a source electrode and a drain electrode included in the transistor 750. The conductive film 772 is formed over the planarization insulating film 770 to function as a pixel electrode, i.e., one electrode of the display element. The conductive film 772 functions as a reflective electrode. The display device 700 in FIG. 22 is what is called a reflective color liquid crystal display device in which external light is reflected by the conductive film 772 to display an image through the coloring film 736.

A conductive film that transmits visible light or a conductive film that reflects visible light can be used as the conductive film 772. For example, a material including one kind selected from indium (In), zinc (Zn), and tin (Sn) is preferably used for the conductive film that transmits visible light. For example, a material including aluminum or silver may be used for the conductive film that reflects visible light. In this embodiment, the conductive film that reflects visible light is used as the conductive film 772.

In the case where a conductive film which reflects visible light is used as the conductive film 772, the conductive film may have a stacked-layer structure. For example, a 100-nm-thick aluminum film is formed as the bottom layer, and a 30-nm-thick silver alloy film (e.g., an alloy film including silver, palladium, and copper) is formed as the top layer. Such a structure makes it possible to obtain the following effects.

(1) Adhesion between the base film and the conductive film 772 can be improved.

(2) The aluminum film and the silver alloy film can be collectively etched depending on a chemical solution.

(3) The conductive film 772 can have a favorable cross-sectional shape (e.g., a tapered shape).

The reason for (3) is as follows: the etching rate of the aluminum film with the chemical solution is lower than that of the silver alloy film, or etching of the aluminum film that is the bottom layer is developed faster than that of the silver alloy film because, when the aluminum film that is the bottom layer is exposed after the etching of the silver alloy film that is the top layer, electrons are extracted from metal that is less noble than the silver alloy film, i.e., aluminum that is metal having a high ionization tendency, and thus etching of the silver alloy film is suppressed.

Note that projections and depressions are provided in part of the planarization insulating film 770 of the pixel portion 702 in the display device 700 in FIG. 22. The projections and depressions can be formed in such a manner that the planarization insulating film 770 is formed using an organic resin film or the like, and projections and depressions are formed on the surface of the organic resin film. The conductive film 772 that functions as a reflective electrode is formed along the projections and depressions. Therefore, when external light is incident on the conductive film 772, the light is reflected diffusely at the surface of the conductive film 772, whereby visibility can be improved.

Note that the display device 700 in FIG. 22 is a reflective color liquid crystal display device given as an example, but a display type is not limited thereto. For example, a transmissive color liquid crystal display device in which the conductive film 772 is a conductive film that transmits visible light may be used. In the case of a transmissive color liquid crystal display device, projections and depressions are not necessarily provided on the planarization insulating film 770.

Although not illustrated in FIG. 22, an alignment film may be provided on a side of the conductive film 772 in contact with the liquid crystal layer 776 and on a side of the conductive film 774 in contact with the liquid crystal layer 776. Although not illustrated in FIG. 22, an optical member (an optical substrate) and the like such as a polarizing member, a retardation member, or an anti-reflection member may be provided as appropriate. For example, circular polarization may be employed by using a polarizing substrate and a retardation substrate. In addition, a backlight, a sidelight, or the like may be used as a light source.

In the case where a liquid crystal element is used as the display element, a thermotropic liquid crystal, a low-molecular liquid crystal, a high-molecular liquid crystal, a polymer-dispersed liquid crystal, a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, or the like can be used. Such a liquid crystal material exhibits a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions.

Alternatively, in the case of employing a horizontal electric field mode, a liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used. A blue phase is one of liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase while temperature of cholesteric liquid crystal is increased. Since the blue phase appears only in a narrow temperature range, a liquid crystal composition in which several weight percent or more of a chiral material is mixed is used for the liquid crystal layer in order to improve the temperature range. The liquid crystal composition which includes liquid crystal exhibiting a blue phase and a chiral material has high response speed. Furthermore, the liquid crystal is optically isotropic, which makes the alignment process unneeded and the viewing angle dependence small. Since an alignment film is not need, rubbing treatment is also unnecessary; accordingly, electrostatic discharge damage caused by the rubbing treatment can be prevented

In the case where a liquid crystal element is used as the display element, a twisted nematic (TN) mode, an in-plane-switching (IPS) mode, a fringe field switching (FFS) mode, an axially symmetric aligned micro-cell (ASM) mode, an optical compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, or the like can be used.

Furthermore, a normally black liquid crystal display device such as a transmissive liquid crystal display device utilizing a vertical alignment (VA) mode may also be used. There are some examples of a vertical alignment mode; for example, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, or an advanced super view (ASV) mode can be employed.

Display Device Using Light-Emitting Element as Display Element

The display device 700 illustrated in FIG. 23 includes a light-emitting element 782. The light-emitting element 782 includes a conductive film 784, an EL layer 786, and a conductive film 788. The display device 700 in FIG. 23 is capable of displaying an image by light emission from the EL layer 786 included in the light-emitting element 782.

The conductive film 784 is connected to the conductive film that functions as a source electrode and a drain electrode included in the transistor 750. The conductive film 784 is formed over the planarization insulating film 770 to function as a pixel electrode, i.e., one electrode of the display element. A conductive film which transmits visible light or a conductive film which reflects visible light can be used as the conductive film 784. The conductive film which transmits visible light can be formed using a material including one kind selected from indium (In), zinc (Zn), and tin (Sn), for example. The conductive film which reflects visible light can be formed using a material including aluminum or silver, for example.

In the display device 700 in FIG. 23, an insulating film 730 is provided over the planarization insulating film 770 and the conductive film 784. The insulating film 730 covers part of the conductive film 784. Note that the light-emitting element 782 has a top emission structure. Therefore, the conductive film 788 has a light-transmitting property and transmits light emitted from the EL layer 786. Although the top-emission structure is described as an example in this embodiment, one embodiment of the present invention is not limited thereto. A bottom-emission structure in which light is emitted to the conductive film 784 side, or a dual-emission structure in which light is emitted to both the conductive film 784 side and the conductive film 788 side may be employed.

The coloring film 736 is provided to overlap with the light-emitting element 782, and the light-blocking film 738 is provided to overlap with the insulating film 730 and to be included in the lead wiring portion 711 and in the source driver circuit portion 704. The coloring film 736 and the light-blocking film 738 are covered with the insulating film 734. A space between the light-emitting element 782 and the insulating film 734 is filled with a sealing film 732. Although a structure with the coloring film 736 is described as the display device 700 in FIG. 23, the structure is not limited thereto. In the case where the EL layer 786 is formed by a separate coloring method, the coloring film 736 is not necessarily provided.

Example of Pixel Configuration for Achieving Color Display

Here, examples of a pixel configuration for achieving color display are described with reference to FIGS. 24A and 24B and FIGS. 25A and 25B. Note that the case where a light-emitting element exhibiting white light emission is used as a light source is described below. FIGS. 24A and 24B and FIGS. 25A and 25B are enlarged plan views of a region 870 of the pixel portion 702 illustrated in FIG. 21. For example, as illustrated in FIG. 24A, three pixels 830 function as subpixels and are collectively used as one pixel 840. The use of a red coloring layer, a green coloring layer, and a blue coloring layer for the three pixels 830 enables full-color display. In FIG. 24A, the pixel 830 emitting red light is denoted by a pixel 830R, the pixel 830 emitting green light is denoted by a pixel 830G, and the pixel 830 emitting blue light is denoted by a pixel 830B. The colors of the coloring layers may be a color other than red, green, and blue; for example, the color may be yellow, cyan, or magenta.

Alternatively, as illustrated in FIG. 24B, four pixels 830 may function as subpixels and may be collectively used as one pixel 840. For example, the colors of the coloring layers corresponding to the four pixels 830 may be red, green, blue, and yellow. In FIG. 24B, the pixel 830 emitting red light, the pixel 830 emitting green light, the pixel 830 emitting blue light, and the pixel 830 emitting yellow light are illustrated as a pixel 830R, a pixel 830G, a pixel 830B, and a pixel 830Y, respectively. By increasing the number of pixels 830 used as one pixel 840, the color reproducibility can be improved. Thus, the display quality of the display device can be improved.

Alternatively, the colors of the coloring layers corresponding to the four pixels 830 may be red, green, blue, and white (see FIG. 24B). With the pixel 830 emitting white light (a pixel 830W), the luminance of the display region can be increased. Note that in the case where the pixel 830W emitting white light is provided, it is not necessary to provide a coloring layer for the pixel 830W. Without the coloring layer for the pixel 830W, there is no luminance reduction at the time of transmitting light through the coloring layer; thus, the luminance of the display region can be increased. Moreover, power consumption of the display device can be reduced. On the other hand, color temperature of white light can be controlled with the coloring layer for the pixel 830W. Thus, the display quality of the display device can be improved. Depending on the intended use of the display device, each pixel 830 may function as a subpixel and two pixels 830 may be used as one pixel 840.

In the case where four pixels 830 are collectively used as one pixel 840, the four pixels 830 may be arranged in a matrix as illustrated in FIG. 25B. In the case where four pixels 830 are collectively used as one pixel 840, a pixel emitting cyan light or magenta light may be used instead of the pixel 830Y or the pixel 830W. Furthermore, a plurality of pixels 830 emitting the same color light may be provided in one pixel 840.

Note that the occupation areas of the pixels 830 in the pixel 840 or shapes of the pixels 830 may be the same or different. In addition, arrangement is not limited to stripe arrangement or matrix arrangement. For example, delta arrangement, Bayer arrangement, pentile arrangement, or the like can be employed. FIG. 25A illustrates an example of pentile arrangement.

The structure described in this embodiment can be used in appropriate combination with any of the structures described in the other embodiments.

Embodiment 4

In this embodiment, a display device that includes a semiconductor device of one embodiment of the present invention is described with reference to FIGS. 26A to 26C.

The display device illustrated in FIG. 26A includes a region including pixels of display elements (hereinafter the region is referred to as a pixel portion 502), a circuit portion being provided outside the pixel portion 502 and including a circuit for driving the pixels (hereinafter the portion is referred to as a driver circuit portion 504), circuits each having a function of protecting an element (hereinafter the circuits are referred to as protection circuits 506), and a terminal portion 507. Note that the protection circuits 506 are not necessarily provided.

Part or the whole of the driver circuit portion 504 is preferably formed over a substrate over which the pixel portion 502 is formed, in which case the number of components and the number of terminals can be reduced. When part or the whole of the driver circuit portion 504 is not formed over the substrate over which the pixel portion 502 is formed, the part or the whole of the driver circuit portion 504 can be mounted by COG or tape automated bonding (TAB).

The pixel portion 502 includes a plurality of circuits for driving display elements arranged in X rows (X is a natural number of 2 or more) and Y columns (Y is a natural number of 2 or more) (hereinafter, such circuits are referred to as pixel circuits 501). The driver circuit portion 504 includes driver circuits such as a circuit for supplying a signal (scan signal) to select a pixel (hereinafter, the circuit is referred to as a gate driver 504 a) and a circuit for supplying a signal (data signal) to drive a display element in a pixel (hereinafter, the circuit is referred to as a source driver 504 b).

The gate driver 504 a includes a shift register or the like. The gate driver 504 a receives a signal for driving the shift register through the terminal portion 507 and outputs a signal. For example, the gate driver 504 a receives a start pulse signal, a clock signal, or the like and outputs a pulse signal. The gate driver 504 a has a function of controlling the potentials of wirings supplied with scan signals (hereinafter, such wirings are referred to as scan lines GL_1 to GL_X). Note that a plurality of gate drivers 504 a may be provided to control the scan lines GL_1 to GL_X separately. Alternatively, the gate driver 504 a has a function of supplying an initialization signal. Without being limited thereto, the gate driver 504 a can supply another signal.

The source driver 504 b includes a shift register or the like. The source driver 504 b receives a signal (video signal) from which a data signal is derived, as well as a signal for driving the shift register, through the terminal portion 507. The source driver 504 b has a function of generating a data signal to be written to the pixel circuit 501 which is based on the video signal. In addition, the source driver 504 b has a function of controlling output of a data signal in response to a pulse signal produced by input of a start pulse signal, a clock signal, or the like. Furthermore, the source driver 504 b has a function of controlling the potentials of wirings supplied with data signals (hereinafter such wirings are referred to as data lines DL_1 to DL_Y). Alternatively, the source driver 504 b has a function of supplying an initialization signal. Without being limited thereto, the source driver 504 b can supply another signal.

The source driver 504 b includes a plurality of analog switches, for example. The source driver 504 b can output, as the data signals, signals obtained by time-dividing the video signal by sequentially turning on the plurality of analog switches. The source driver 504 b may include a shift register or the like.

A pulse signal and a data signal are input to each of the plurality of pixel circuits 501 through one of the plurality of scan lines GL supplied with scan signals and one of the plurality of data lines DL supplied with data signals, respectively. Writing and holding of the data signal to and in each of the plurality of pixel circuits 501 are controlled by the gate driver 504 a. For example, to the pixel circuit 501 in the m-th row and the n-th column (m is a natural number of less than or equal to X, and n is a natural number of less than or equal to Y), a pulse signal is input from the gate driver 504 a through the scan line GL_m, and a data signal is input from the source driver 504 b through the data line DL_n in accordance with the potential of the scan line GL_m.

The protection circuit 506 illustrated in FIG. 26A is connected to, for example, the scan line GL between the gate driver 504 a and the pixel circuit 501. Alternatively, the protection circuit 506 is connected to the data line DL between the source driver 504 b and the pixel circuit 501. Alternatively, the protection circuit 506 can be connected to a wiring between the gate driver 504 a and the terminal portion 507. Alternatively, the protection circuit 506 can be connected to a wiring between the source driver 504 b and the terminal portion 507. Note that the terminal portion 507 means a portion having terminals for inputting power, control signals, and video signals to the display device from external circuits.

The protection circuit 506 is a circuit that electrically connects a wiring connected to the protection circuit to another wiring when a potential out of a certain range is applied to the wiring connected to the protection circuit.

As illustrated in FIG. 26A, the protection circuits 506 are provided for the pixel portion 502 and the driver circuit portion 504, so that the resistance of the display device to overcurrent generated by electrostatic discharge (ESD) or the like can be improved. Note that the configuration of the protection circuits 506 is not limited to that, and for example, the protection circuit 506 may be configured to be connected to the gate driver 504 a or the protection circuit 506 may be configured to be connected to the source driver 504 b. Alternatively, the protection circuit 506 may be configured to be connected to the terminal portion 507.

In FIG. 26A, an example in which the driver circuit portion 504 includes the gate driver 504 a and the source driver 504 b is shown; however, the structure is not limited thereto. For example, only the gate driver 504 a may be formed and a separately prepared substrate where a source driver circuit is formed (e.g., a driver circuit substrate formed with a single crystal semiconductor film or a polycrystalline semiconductor film) may be mounted.

Each of the plurality of pixel circuits 501 in FIG. 26A can have the structure illustrated in FIG. 26B, for example.

The pixel circuit 501 illustrated in FIG. 26B includes a liquid crystal element 570, a transistor 550, and a capacitor 560. As the transistor 550, any of the transistors described in the above embodiment, for example, can be used.

The potential of one of a pair of electrodes of the liquid crystal element 570 is set in accordance with the specifications of the pixel circuit 501 as appropriate. The alignment state of the liquid crystal element 570 depends on written data. A common potential may be supplied to one of the pair of electrodes of the liquid crystal element 570 included in each of the plurality of pixel circuits 501. Furthermore, the potential supplied to one of the pair of electrodes of the liquid crystal element 570 in the pixel circuit 501 in one row may be different from the potential supplied to one of the pair of electrodes of the liquid crystal element 570 in the pixel circuit 501 in another row.

As examples of a driving method of the display device including the liquid crystal element 570, any of the following modes can be given: a TN mode, an STN mode, a VA mode, an axially symmetric aligned micro-cell (ASM) mode, an optically compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, an MVA mode, a patterned vertical alignment (PVA) mode, an IPS mode, an FFS mode, a transverse bend alignment (TBA) mode, and the like. Other examples of the driving method of the display device include an electrically controlled birefringence (ECB) mode, a polymer-dispersed liquid crystal (PDLC) mode, a polymer network liquid crystal (PNLC) mode, and a guest-host mode. Note that the present invention is not limited to these examples, and various liquid crystal elements and driving methods can be applied to the liquid crystal element and the driving method thereof.

In the pixel circuit 501 in the m-th row and the n-th column, one of a source electrode and a drain electrode of the transistor 550 is electrically connected to the data line DL_n, and the other is electrically connected to the other of the pair of electrodes of the liquid crystal element 570. A gate electrode of the transistor 550 is electrically connected to the scan line GL_m. The transistor 550 has a function of controlling whether to write a data signal by being turned on or off.

One of a pair of electrodes of the capacitor 560 is electrically connected to a wiring to which a potential is supplied (hereinafter referred to as a potential supply line VL), and the other is electrically connected to the other of the pair of electrodes of the liquid crystal element 570. The potential of the potential supply line VL is set in accordance with the specifications of the pixel circuit 501 as appropriate. The capacitor 560 functions as a storage capacitor for storing written data.

For example, in the display device including the pixel circuit 501 in FIG. 26B, the pixel circuits 501 are sequentially selected row by row by the gate driver 504 a illustrated in FIG. 26A, whereby the transistors 550 are turned on and a data signal is written.

When the transistors 550 are turned off, the pixel circuits 501 in which the data has been written are brought into a holding state. This operation is sequentially performed row by row; thus, an image can be displayed.

Alternatively, each of the plurality of pixel circuits 501 in FIG. 26A can have the structure illustrated in FIG. 26C, for example.

The pixel circuit 501 illustrated in FIG. 26C includes transistors 552 and 554, a capacitor 562, and a light-emitting element 572. Any of the transistors described in the above embodiment, for example, can be used as one or both of the transistors 552 and 554.

One of a source electrode and a drain electrode of the transistor 552 is electrically connected to a wiring to which a data signal is supplied (hereinafter referred to as a signal line DL_n). A gate electrode of the transistor 552 is electrically connected to a wiring to which a gate signal is supplied (hereinafter referred to as a scan line GL_m).

The transistor 552 has a function of controlling whether to write a data signal by being turned on or off.

One of a pair of electrodes of the capacitor 562 is electrically connected to a wiring to which a potential is supplied (hereinafter referred to as a potential supply line VL_a), and the other is electrically connected to the other of the source electrode and the drain electrode of the transistor 552.

The capacitor 562 functions as a storage capacitor for storing written data.

One of a source electrode and a drain electrode of the transistor 554 is electrically connected to the potential supply line VL_a. Furthermore, a gate electrode of the transistor 554 is electrically connected to the other of the source electrode and the drain electrode of the transistor 552.

One of an anode and a cathode of the light-emitting element 572 is electrically connected to a potential supply line VL_b, and the other is electrically connected to the other of the source electrode and the drain electrode of the transistor 554.

As the light-emitting element 572, an organic electroluminescent element (also referred to as an organic EL element) can be used, for example. Note that the light-emitting element 572 is not limited to an organic EL element; an inorganic EL element including an inorganic material may be used.

A high power supply potential VDD is supplied to one of the potential supply line VL_a and the potential supply line VL_b, and a low power supply potential VSS is supplied to the other.

For example, in the display device including the pixel circuit 501 in FIG. 26C, the pixel circuits 501 are sequentially selected row by row by the gate driver 504 a illustrated in FIG. 26A, whereby the transistors 552 are turned on and a data signal is written.

When the transistors 552 are turned off, the pixel circuits 501 in which the data has been written are brought into a holding state. Furthermore, the amount of current flowing between the source electrode and the drain electrode of the transistor 554 is controlled in accordance with the potential of the written data signal. The light-emitting element 572 emits light with luminance corresponding to the amount of flowing current. This operation is sequentially performed row by row; thus, an image can be displayed.

Two or more transistors may be used in the pixel circuit 501 in order to correct the threshold voltage of a transistor connected to a light-emitting element. For example, FIGS. 27A and 27B illustrate examples in which five transistors are used in the pixel circuit 501.

The pixel circuit 501 illustrated in FIGS. 27A and 27B includes transistors 650 to 654, a capacitor 655, and a light-emitting element 656. Note that FIGS. 27A and 27B illustrate the case where the transistors 650 to 654 are n-channel transistors.

In FIGS. 27A and 27B, a wiring GLa, a wiring GLb, and a wiring GLc are electrically connected to the gate driver 504 a, and a wiring SL is electrically connected to the source driver 504 b.

First, the pixel circuit 501 illustrated in FIG. 27A is described.

In FIG. 27A, the transistor 651 has a function of choosing conduction or non-conduction between the wiring SL and one of a pair of electrodes of the capacitor 655. The other of the pair of electrodes of the capacitor 655 is connected to one of a source and a drain of the transistor 650. The transistor 652 has a function of choosing conduction or non-conduction between a wiring IL and a gate of the transistor 650. The transistor 653 has a function of choosing conduction or non-conduction between the one of the pair of electrodes of the capacitor 655 and the gate of the transistor 650. The transistor 654 has a function of choosing conduction or non-conduction between the one of the source and the drain of the transistor 650 and an anode of the light-emitting element 656. A cathode of the light-emitting element 656 is electrically connected to a wiring CL.

In FIG. 27A, the other of the source and the drain of the transistor 650 is connected to a wiring VL.

In FIG. 27A, whether the transistor 651 is turned on or off is determined by the potential of the wiring GLa connected to the gate of the transistor 651. Whether the transistor 652 is turned on or off is determined by the potential of the wiring GLa connected to the gate of the transistor 652. Whether the transistor 653 is turned on or off is determined by the potential of the wiring GLb connected to the gate of the transistor 653. Whether the transistor 654 is turned on or off is determined by the potential of the wiring GLc connected to the gate of the transistor 654.

Next, the pixel circuit 501 illustrated in FIG. 27B is described.

In FIG. 27B, the transistor 651 has a function of choosing conduction or non-conduction between the wiring SL and one of a pair of electrodes of the capacitor 655. The other of the pair of electrodes of the capacitor 655 is connected to one of the source and the drain of the transistor 650 and the anode of the light-emitting element 656. The transistor 652 has a function of choosing conduction or non-conduction between a wiring IL and a gate of the transistor 650. The transistor 653 has a function of choosing conduction or non-conduction between the one of the pair of electrodes of the capacitor 655 and the gate of the transistor 650. The transistor 654 has a function of choosing conduction or non-conduction between the one of the source and the drain of the transistor 650 and the anode of the light-emitting element 656 and the wiring RL. The other of the source and the drain of the transistor 650 is connected to the wiring VL.

In FIG. 27B, whether the transistor 651 is turned on or off is determined by the potential of the wiring GLa connected to the gate of the transistor 651. Whether the transistor 652 is turned on or off is determined by the potential of the wiring GLa connected to the gate of the transistor 652. Whether the transistor 653 is turned on or off is determined by the potential of the wiring GLb connected to the gate of the transistor 653. Whether the transistor 654 is turned on or off is determined by the potential of the wiring GLc connected to the gate of the transistor 654.

It is preferable to use the transistor described in Embodiment 1 as a transistor used in the pixel circuit 501 illustrated in FIG. 26C and FIGS. 27A and 27B. In particular, the transistor described in Embodiment 1 is preferably used as the transistor connected to a light-emitting element among the above-described transistors. The transistor described in Embodiment 1 exhibits good Id-Vd saturation characteristics; accordingly, even when drain voltage applied to the transistor varies, a constant current can be applied to the light-emitting element. Thus, the use of the transistor described in Embodiment 1 leads to provide a display device that is less likely to cause variations in luminance of light-emitting elements.

The structure described in this embodiment can be used in appropriate combination with the structure described in any of the other embodiments.

Embodiment 5

In this embodiment, a display module and electronic appliances that include a semiconductor device of one embodiment of the present invention is described with reference to FIG. 28 and FIGS. 29A to 29G.

In a display module 8000 illustrated in FIG. 28, a touch panel 8004 connected to an FPC 8003, a display panel 8006 connected to an FPC 8005, a backlight 8007, a frame 8009, a printed board 8010, and a battery 8011 are provided between an upper cover 8001 and a lower cover 8002.

The semiconductor device of one embodiment of the present invention can be used for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002 can be changed as appropriate in accordance with the sizes of the touch panel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitive touch panel and can be formed to overlap with the display panel 8006. A counter substrate (sealing substrate) of the display panel 8006 can have a touch panel function. A photosensor may be provided in each pixel of the display panel 8006 to form an optical touch panel.

The backlight 8007 includes a light source 8008. Note that although a structure in which the light sources 8008 are provided over the backlight 8007 is illustrated in FIG. 28, one embodiment of the present invention is not limited to this structure. For example, a structure in which the light source 8008 is provided at an end portion of the backlight 8007 and a light diffusion plate is further provided may be employed. Note that the backlight 8007 need not be provided in the case where a self-luminous light-emitting element such as an organic EL element is used or in the case where a reflective panel or the like is employed.

The frame 8009 protects the display panel 8006 and also functions as an electromagnetic shield for blocking electromagnetic waves generated by the operation of the printed board 8010. The frame 8009 may function as a radiator plate.

The printed board 8010 is provided with a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal. As a power source for supplying power to the power supply circuit, an external commercial power source or a power source using the battery 8011 provided separately may be used. The battery 8011 can be omitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet.

FIGS. 29A to 29G illustrate electronic appliances. These electronic appliances can include a housing 9000, a display portion 9001, a speaker 9003, operation keys 9005 (including a power switch or an operation switch), a connection terminal 9006, a sensor 9007 (a sensor having a function of measuring or sensing force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared ray), a microphone 9008, and the like.

The electronic appliances illustrated in FIGS. 29A to 29G can have a variety of functions. Examples of the functions are a function of displaying a variety of data (a still image, a moving image, a text image, and the like) on the display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of controlling a process with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, and a function of reading a program or data stored in a memory medium and displaying the program or data on the display portion. Note that functions that can be provided for the electronic appliances illustrated in FIGS. 29A to 29G are not limited to those described above, and the electronic appliances can have a variety of functions. Although not illustrated in FIGS. 29A to 29G, the electronic appliance may include a plurality of display portions. Furthermore, the electronic appliance may be provided with a camera and the like and have a function of shooting a still image, a function of shooting a moving image, a function of storing a shot image in a memory medium (an external memory medium or a memory medium incorporated in the camera), a function of displaying a shot image on the display portion, or the like.

The electronic appliances illustrated in FIGS. 29A to 29G are described in detail below.

FIG. 29A is a perspective view illustrating a portable information terminal 9100. A display portion 9001 of the portable information terminal 9100 is flexible. Therefore, the display portion 9001 can be incorporated along a bent surface of a bent housing 9000. Furthermore, the display portion 9001 includes a touch sensor, and operation can be performed by touching the screen with a finger, a stylus, or the like. For example, by touching an icon displayed on the display portion 9001, application can be started.

FIG. 29B is a perspective view illustrating a portable information terminal 9101. The portable information terminal 9101 functions as, for example, one or more of a telephone set, a notebook, and an information browsing system. Specifically, the portable information terminal 9101 can be used as a smartphone. Note that although the speaker 9003, the connection terminal 9006, the sensor 9007, and the like of the portable information terminal 9101 are not illustrated in FIG. 29B, they can be provided in the same positions as the portable information terminal 9100 in FIG. 29A. The portable information terminal 9101 can display characters and image information on its plurality of surfaces. For example, three operation buttons 9050 (also referred to as operation icons or simply icons) can be displayed on one surface of the display portion 9001. Furthermore, information 9051 indicated by dashed rectangles can be displayed on another surface of the display portion 9001. Examples of the information 9051 include display indicating reception of an incoming email, social networking service (SNS) message, and call; the title and sender of an email and SNS massage; the date; the time; remaining battery; and the reception strength of an antenna. Alternatively, the operation buttons 9050 or the like may be displayed in place of the information 9051.

FIG. 29C is a perspective view illustrating a portable information terminal 9102. The portable information terminal 9102 has a function of displaying information, for example, on three or more sides of the display portion 9001. Here, information 9052, information 9053, and information 9054 are displayed on different sides. For example, a user of the portable information terminal 9102 can see the display (here, the information 9053) with the portable information terminal 9102 put in a breast pocket of his/her clothes. Specifically, a caller's phone number, name, or the like of an incoming call is displayed in a position that can be seen from above the portable information terminal 9102. Thus, the user can see the display without taking out the portable information terminal 9102 from the pocket and decide whether to answer the call.

FIG. 29D is a perspective view illustrating a wrist-watch-type portable information terminal 9200. The portable information terminal 9200 is capable of executing a variety of applications such as mobile phone calls, e-mailing, reading and editing texts, music reproduction, Internet communication, and a computer game. The display surface of the display portion 9001 is bent, and images can be displayed on the bent display surface. The portable information terminal 9200 can employ near field communication that is a communication method based on an existing communication standard. In that case, for example, mutual communication between the portable information terminal 9200 and a headset capable of wireless communication can be performed, and thus hands-free calling is possible. Moreover, the portable information terminal 9200 includes the connection terminal 9006, and data can be directly transmitted to and received from another information terminal via a connector. Charging through the connection terminal 9006 is possible. Note that the charging operation may be performed by wireless power feeding without using the connection terminal 9006.

FIGS. 29E, 29F, and 29G are perspective views each illustrating a foldable portable information terminal 9201. FIG. 29E is a perspective view illustrating the portable information terminal 9201 that is opened, FIG. 29F is a perspective view illustrating the portable information terminal 9201 that is being opened or being folded, and FIG. 29G is a perspective view illustrating the portable information terminal 9201 that is folded. The portable information terminal 9201 is highly portable when folded. When the portable information terminal 9201 is opened, a seamless large display region is highly browsable. The display portion 9001 of the portable information terminal 9201 is supported by three housings 9000 joined together by hinges 9055. By folding the portable information terminal 9201 at a connection portion between two housings 9000 with the hinges 9055, the portable information terminal 9201 can be reversibly changed in shape from an opened state to a folded state. For example, the portable information terminal 9201 can be bent with a radius of curvature of greater than or equal to 1 mm and less than or equal to 150 mm.

The electronic appliances described in this embodiment each include the display portion for displaying some sort of data. Note that the semiconductor device of one embodiment of the present invention can also be used for an electronic appliance that does not have a display portion. The structure in which the display portion of the electronic appliance described in this embodiment is flexible and display can be performed on the bent display surface or the structure in which the display portion of the electronic appliance is foldable is described as an example; however, the structure is not limited thereto and a structure in which the display portion of the electronic appliance is not flexible and display is performed on a plane portion may be employed.

The structure described in this embodiment can be used in appropriate combination with any of the structures described in the other embodiments.

Example 1

In this example, the transistors described in Embodiment 1 were fabricated, and the transistor characteristics were measured.

FIG. 30 shows the characteristics of a transistor A fabricated in this example. The transistor A corresponds to the one in which the conductive film 120 b and the opening 142 a in the transistor 130 illustrated in FIGS. 3A to 3C are omitted. The channel length (L3) of the transistor A was 10 μm, and the channel width (W3) thereof was 59.7 μm. As the oxide semiconductor film 108, a 35-nm-thick oxide semiconductor film was formed by a sputtering method using an In—Ga—Zn metal oxide target with an atomic ratio of In to Ga to Zn being 1:1:1. For a method of forming other components of the transistor A, the method of manufacturing a semiconductor device described in Embodiment 1 can be referred to.

FIG. 30 shows the Id-Vd characteristics and the Id-Vg (gate voltage) characteristics in the case where an inner electrode (the conductive film 112 b in FIGS. 3A to 3C) is used as a drain of the transistor A (Inner-Drain) and in the case where an outer electrode (the conductive film 112 a in FIGS. 3A to 3C) is used as the drain of the transistor A (Outer-Drain).

The Id-Vd characteristics shown in FIG. 30 were obtained by measuring drain current under the conditions in which the gate voltage was set to 2 V, 4 V, 6 V, 8 V, and 10 V and the drain voltage was swept from 0 V to 20 V.

The Id-Vg characteristics shown in FIG. 30 were obtained by measuring drain current under the conditions in which the drain voltage was set to 0.1 V and 10 V and the gate voltage was swept from −15 V to 15 V. In addition, the graphs showing the Id-Vg characteristics also show the field-effect mobility (μ_(FE)) (dotted lines in FIG. 30).

From FIG. 30, it was found that when the outer electrode was used as the drain of the transistor A, a negative shift in threshold (channel length modulation effect) due to drain voltage can be small and good Id-Vd saturation characteristics in which the drain current is constant can be obtained.

FIG. 31 shows the characteristics of a transistor B fabricated in this example. The transistor B corresponds to the one in which the conductive film 120 b and the opening 142 a in the transistor 140 illustrated in FIGS. 4A to 4C are omitted. The channel length (L4) of the transistor B was 10 μm, and the channel width (W4) thereof was 76 μm. As the oxide semiconductor film 108, a 35-nm-thick oxide semiconductor film was formed by a sputtering method using an In—Ga—Zn metal oxide target with an atomic ratio of In to Ga to Zn being 1:1:1. For a method of forming other components of the transistor B, the method of manufacturing a semiconductor device described in Embodiment 1 can be referred to.

Similarly to FIG. 30, FIG. 31 shows the Id-Vd characteristics and the Id-Vg characteristics in the case where an inner electrode (the conductive film 112 b in FIGS. 4A to 4C) is used as a drain of the transistor B (Inner-Drain) and in the case where an outer electrode (the conductive film 112 a in FIGS. 4A to 4C) is used as the drain of the transistor A (Outer-Drain). For conditions for measuring the characteristics, the above description of FIG. 30 can be referred to.

From FIG. 31, it was found that when the outer electrode was used as the drain of the transistor B, a negative shift in threshold (channel length modulation effect) due to drain voltage can be small and good Id-Vd saturation characteristics in which the drain current is constant can be obtained.

FIG. 32 shows the characteristics of a transistor C and a transistor D fabricated in this example.

The transistor C corresponds to the one in which the conductive film 120 b and the openings 142 a and 142 b in the transistor 100 illustrated in FIGS. 1A to 1C are omitted. The channel length (L1) of the transistor C was 10 μm, and the channel width (W1) thereof was 50 μm. As the oxide semiconductor film 108, a 35-nm-thick oxide semiconductor film was formed by a sputtering method using an In—Ga—Zn metal oxide target with an atomic ratio of In to Ga to Zn being 1:1:1. For a method of forming other components of the transistor C, the method of manufacturing a semiconductor device described in Embodiment 1 can be referred to.

The transistor D is the same as the transistor 100 illustrated in FIGS. 1A to 1C. The channel length (L1) of the transistor D was 10 μm, and the channel width (W1) thereof was 50 μm. As the oxide semiconductor film 108, a 35-nm-thick oxide semiconductor film was formed by a sputtering method using an In—Ga—Zn metal oxide target with an atomic ratio of In to Ga to Zn being 1:1:1. For a method of forming other components of the transistor D, the method of manufacturing a semiconductor device described in Embodiment 1 can be referred to.

For the conditions for measuring the Id-Vg characteristics and the Id-Vg characteristics shown in FIG. 32, description of FIG. 30 can be referred to.

FIG. 32 shows that the transistor D including the second gate electrode (the conductive film 120 b) has a high on-state current and exhibits good Id-Vd saturation characteristics in which drain current is constant, as compared with the transistor C.

FIG. 33 shows the characteristics of a transistor E fabricated in this example.

The transistor E is the same as the transistor 100 illustrated in FIGS. 1A to 1C. The channel length (L1) of the transistor E was 10 μm, and the channel width (W1) thereof was 50 μm. The oxide semiconductor film 108 was formed by a sputtering method using an In—Ga—Zn metal oxide target with an atomic ratio of In to Ga to Zn being 3:1:2. In this example, the thickness (OS thickness) of the oxide semiconductor film 108 in the transistor E was set to 10 nm, 25 nm, and 40 nm. For a method of forming other components of the transistor E, the method of manufacturing a semiconductor device described in Embodiment 1 can be referred to.

Note that the thickness of an oxide semiconductor (OS thickness) shown in FIG. 33 refers to a thickness of a channel region of the transistor E. That is, the OS thickness corresponds to a thickness of the oxide semiconductor film 108 in a portion which is in contact with neither the conductive film 112 a nor the conductive film 112 b, in the cross-sectional view in FIG. 1B.

For the conditions for measuring the Id-Vg characteristics and the Id-Vg characteristics shown in FIG. 33, description of FIG. 30 can be referred to.

The results in FIG. 33 indicate that the thinner the thickness of the oxide semiconductor is, the more improved the on-state current and the field-effect mobility of the transistor E are. In the case where the oxide semiconductor has a thickness of 40 nm, the field-effect mobility was approximately 11 cm²/Vs. In the case where the oxide semiconductor has a thickness of 25 nm, the field-effect mobility was approximately 15 cm²/Vs. In the case where the oxide semiconductor has a thickness of 10 nm, the field-effect mobility was approximately 19 cm²/Vs.

FIG. 34 shows the characteristics of a transistor F fabricated in this example.

The channel length (L1) of the transistor F was set to 2 μm. The other conditions of the transistor F were the same as the conditions of the transistor E. As in the case of the transistor E, the thickness (OS thickness) of the oxide semiconductor film 108 in the transistor F was set to 10 nm, 25 nm, and 40 nm.

Note that the thickness of an oxide semiconductor (OS thickness) shown in FIG. 34 refers to a thickness of a channel region of the transistor F. That is, the OS thickness corresponds to a thickness of the oxide semiconductor film 108 in a portion which is in contact with neither the conductive film 112 a nor the conductive film 112 b, in the cross-sectional view in FIG. 1B.

For the conditions for measuring the Id-Vg characteristics and the Id-Vg characteristics shown in FIG. 34, description of FIG. 30 can be referred to.

The results in FIG. 34 indicate that the thinner the oxide semiconductor is, the better Id-Vd saturation characteristics in which drain voltage is constant becomes. In particular, it was found that good saturation characteristics can be obtained when the oxide semiconductor film has a thickness of 10 nm.

From the results in FIG. 33 and FIG. 34, it was found that the thickness of the oxide semiconductor used for a channel region is preferably less than or equal to 25 nm, more preferably less than or equal to 10 nm.

Example 2

In this example, the transistors described in Embodiment 1 were fabricated, the transistor characteristics were measured, and the channel length modulation coefficient was calculated.

FIG. 35 shows the characteristics of a transistor G and a transistor H fabricated in this example.

The transistor G corresponds to the one in which the conductive film 120 b and the opening 142 a in the transistor 130 illustrated in FIGS. 3A to 3C are omitted. That is, the transistor G does not include the second gate electrode. The channel length (L3) of the transistor G was 6 μm, and the channel width (W3) thereof was 59.7 μm. The oxide semiconductor film 108 was formed by a sputtering method using an In—Ga—Zn metal oxide target with an atomic ratio of In to Ga to Zn being 1:1:1. The thickness of the oxide semiconductor film 108 was set to 20 nm. For a method of forming other components of the transistor G, the method of manufacturing a semiconductor device described in Embodiment 1 can be referred to.

The transistor 130 illustrated in FIGS. 3A to 3C was fabricated as the transistor H. The transistor H is an s-channel transistor including a conductive film 104 that functions as a first gate electrode and the conductive film 120 b that functions as a second gate electrode. For the other details of the transistor H, description of the transistor G can be referred to.

FIG. 35 shows the Id-Vd characteristics of the transistor G and the transistor H. Similarly to FIG. 30, FIG. 35 shows the Id-Vd characteristics in the case of Inner-Drain and those in the case of Outer-Drain. The drain current was measured under the conditions in which the gate voltage was set to 2 V, 4 V, 6 V, 8 V, and 10 V and the drain voltage was swept from 0 V to 20 V.

A channel length modulation coefficient (2) is shown in each graph. The channel length modulation coefficient was calculated using Formula (1) below. Specifically, δId/δVd was calculated from a difference in drain current between when Vd=10V, Vg=6 V and when Vd=16V, Vg=6 V, and the obtained value was multiplied by the reciprocal of drain current (1/Id) when Vd=16V, whereby the value of λ was obtained. The smaller the value of λ is, the smaller the channel length modulation effect is, resulting in good Id-Vd saturation characteristics.

λ=δId/δVd×1/Id  (1)

The results in FIG. 35 indicate that the saturation characteristics are more improved in the transistor H with the s-channel structure than in the transistor G without the s-channel structure. As in Example 1, the transistors with Outer-Drain have better saturation characteristics than the transistors with Inner-Drain. However, the transistor H with Inner-Drain has a small channel length modulation coefficient and exhibits good saturation characteristics.

FIG. 36 shows the characteristics of a transistor I and a transistor J fabricated in this example.

The transistor I corresponds to a transistor in which the oxide semiconductor film 108 in the transistor G was formed by a sputtering method using an In—Ga—Zn metal oxide target with an atomic ratio of In to G to Zn being 4:2:4.1. For a method of forming other components of the transistor I, the description of the transistor G can be referred to.

The transistor J corresponds to a transistor in which the oxide semiconductor film 108 in the transistor H was formed by a sputtering method using an In—Ga—Zn metal oxide target with an atomic ratio of In to G to Zn being 4:2:4.1. For a method of forming other components of the transistor J, the description of the transistor H can be referred to.

FIG. 36 shows the Id-Vd characteristics of the transistor I and the transistor J. The detailed measurement conditions are the same as those for the transistors G and H.

Similarly to the results in FIG. 35, the results in FIG. 36 indicate that the saturation characteristics are more improved in the transistor J with the s-channel structure than in the transistor I without the s-channel structure. It was also found that the transistor J has a small channel length modulation coefficient and exhibits good saturation characteristics even in the case of Inner-Drain.

When the oxide semiconductor film 108 is formed using an In—Ga—Zn metal oxide target in which the content of In is larger than the content of Ga, the saturation characteristics of the transistor becomes worse; however, the saturation characteristics are improved in a transistor with the s-channel structure described in Embodiment 1.

This application is based on Japanese Patent Application serial no. 2014-128673 filed with Japan Patent Office on Jun. 23, 2014, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A semiconductor device comprising: a first gate electrode over a substrate; a first gate insulating film over the first gate electrode; an oxide semiconductor film over the first gate insulating film, the oxide semiconductor film comprising a first region and a second region; a source electrode and a drain electrode over the oxide semiconductor film; a second gate insulating film over the oxide semiconductor film, the source electrode, and the drain electrode; and a second gate electrode over the second gate insulating film, the second gate electrode being electrically connected to the first gate electrode, wherein the first region overlaps with one of the source electrode and the drain electrode and comprises a first edge, wherein the second region overlaps with the other of the source electrode and the drain electrode and comprises a second edge opposed to the first edge, and wherein a length of the first edge is shorter than a length of the second edge when seen from above.
 2. The semiconductor device according to claim 1, wherein the first region overlaps with the source electrode, and wherein the second region overlaps with the drain electrode.
 3. A semiconductor device comprising: a first gate electrode over a substrate; a first gate insulating film over the first gate electrode; an oxide semiconductor film over the first gate insulating film, the oxide semiconductor film comprising a first region and a second region; a source electrode and a drain electrode over the oxide semiconductor film; a second gate insulating film over the oxide semiconductor film, the source electrode, and the drain electrode; and a second gate electrode over the second gate insulating film, the second gate electrode being electrically connected to the first gate electrode, wherein the first region overlaps with one of the source electrode and the drain electrode and comprises a first edge, wherein the second region overlaps with the other of the source electrode and the drain electrode and comprises a second edge opposed to the first edge, wherein a length of the first edge is shorter than a length of the second edge when seen from above, and wherein the first region is surrounded by the second region when seen from above.
 4. The semiconductor device according to claim 3, wherein the first region overlaps with the source electrode, and wherein the second region overlaps with the drain electrode.
 5. The semiconductor device according to claim 3, wherein the first gate electrode is electrically connected to the second gate electrode through an opening provided in the first gate insulating film and the second gate insulating film, and wherein a side surface of the oxide semiconductor film faces the second gate electrode provided in the opening.
 6. The semiconductor device according to claim 3, wherein each of the first edge and the second edge has a circular shape or a quadrangular shape when seen from above.
 7. The semiconductor device according to claim 3, wherein a thickness of the oxide semiconductor film is greater than 0 nm and less than or equal to 20 nm.
 8. The semiconductor device according to claim 3, wherein the oxide semiconductor film comprises a crystal part.
 9. The semiconductor device according to claim 3, wherein the oxide semiconductor film comprises In, Zn, and M, wherein M is selected from the group consisting of Ti, Ga, Y, Zr, Sn, La, Ce, Nd, and Hf, and wherein the oxide semiconductor film comprises a region in which a content of In is larger than a content of M.
 10. An electronic device including the semiconductor device according to claim claim
 3. 11. A semiconductor device comprising: a first gate electrode over a substrate; a first gate insulating film over the first gate electrode; an oxide semiconductor film over the first gate insulating film, the oxide semiconductor film comprising a first region and a second region; a source electrode and a drain electrode over the oxide semiconductor film; a second gate insulating film over the oxide semiconductor film, the source electrode, and the drain electrode; and a second gate electrode over the second gate insulating film, the second gate electrode being electrically connected to the first gate electrode, wherein the first region overlaps with one of the source electrode and the drain electrode and comprises a first edge, wherein the second region overlaps with the other of the source electrode and the drain electrode and comprises a second edge opposed to the first edge, wherein a length of the first edge is shorter than a length of the second edge when seen from above, and wherein the oxide semiconductor film has a fan shape or a trapezoidal shape when seen from above.
 12. The semiconductor device according to claim 11, wherein the first region overlaps with the source electrode, and wherein the second region overlaps with the drain electrode.
 13. The semiconductor device according to claim 11, wherein the first gate electrode is electrically connected to the second gate electrode through an opening provided in the first gate insulating film and the second gate insulating film, and wherein a side surface of the oxide semiconductor film faces the second gate electrode provided in the opening.
 14. The semiconductor device according to claim 11, wherein the oxide semiconductor film is surrounded by the first gate electrode and the second gate electrode in a channel width direction.
 15. The semiconductor device according to claim 11, wherein a thickness of the oxide semiconductor film is greater than 0 nm and less than or equal to 20 nm.
 16. The semiconductor device according to claim 11, wherein the oxide semiconductor film comprises a crystal part.
 17. The semiconductor device according to claim 11, wherein the oxide semiconductor film comprises In, Zn, and M, wherein M is selected from the group consisting of Ti, Ga, Y, Zr, Sn, La, Ce, Nd, and Hf, and wherein the oxide semiconductor film comprises a region in which a content of In is larger than a content of M.
 18. An electronic device including the semiconductor device according to claim
 11. 